Sexual orientation is not easy to define. However, according to many references, sexual orientation may be explained as the direction of an individual's sexuality, usually conceived according to the sex (sexual identity) or gender of the persons whom the individual finds sexually attractive and can interact with socially and psychologically (gender identity). The most commonly used categories of sexual orientation are heterosexuality (being sexually attracted to members of the opposite sex), homosexuality (being sexually attracted to members of the same sex) and bisexuality (being sexually attracted to members of either sex). In addition to these categories, we do have a fourth one: intersexuality. This latter describes those who possess anatomical, psychological, and/or genetic characteristics that correspond to both male and female with variable degrees of maleness and femaleness.

For some people, sexual orientation is continuous and fixed throughout their lives. For others, sexual orientation may be fluid and change over time. This is why sexual orientation is generally difficult to quantify due to its diversity that starts from self-labeling, to sexual behavior, sexual fantasy and patterns of erotic arousal.
However, starting the 19th century many attempts tried to establish scales that measure sexual orientation. The first one referred to the German physician and sexologist Magnus Hirschfield who measured the strength of an individual's sexual desire on two independent 10-point scales.
Fifty years later, the American sexologist Alfred Kinsey established his own mono-dimensional scale in order to estimate sexual orientation in humans: from 0 (exclusively heterosexual) to 6 (exclusively homosexual), with an additional category, X, for those with no sexual attraction to either women or men.  
In 1985, Frits Klein developed a scale to test his theory about the dynamic and multi-variable process of sexual orientation. In his book The Bisexual Option, Fritz Klein suggests that sexual orientation is dynamic and may change over time and is multi-variable indeed, i.e. it is composed of various elements, both sexual and non-sexual. He took into consideration sexual attraction, sexual behavior, sexual fantasies, emotional and social partners, lifestyle, and self-identification. Each of these variables was measured for the person's past, present, and ideal.

Sexual orientation probably is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences. In other words, it is a complex interaction of environmental, cognitive and biological factors. However, many studies pointed out that certain genes and/or hormonal exposure increase the tendency to homosexuality with all its derivatives.

Lesbianism is known since old times: in the Old Testament, books, literature, etc… The first known lesbian was a poet named Sapho de Lesbos (600 B.C) who wrote many poems that reflect lesbian love, jealousy, and hate. Marguerite Yourcenar (1903-1987) was a lesbian who lived 42 years in love with Grace Frick (an American teacher). Yourcenar wrote many famous books like "Le Jardin des Chimères", "Alexis ou le Traité du vain combat”, etc.. and met in 1937 the famous American lesbian Virginia Woolf for whom she translated "The Waves". Lesbianism is not limited to literature and many lesbians are gifted in other fields such as Frida Kahlo (1907-1954) the famous lesbian painter, Catherine Lara, Tracy Chapman, Skunk Anansi, Samantha Fox famous lesbian singers, Marlène Dietrich, Greta Garbo, Jodie Foster, Cynthia Nixon famous actors, and Martina Navratilova, Amélie Mauresmo, Sheryl Swoopes famous sportswomen.
Many recent movies involved lesbian actors and messages that support and/or encourage lesbianism. This is the case of the movies De Bound (1996), Fucking Amal (1998), Mudholland (2000), L Word (2004), etc…

Asexuality is a general term or self-designation for people who do not exhibit sexual attraction, or who otherwise find sexual behavior unappealing.

There is debate as to whether this is a sexual dysfunction or a sexual orientation. Furthermore, there is disagreement over the exact definition of the word. The term is sometimes used as a gender identity by those who believe their lack of sexual attraction places them outside the traditional definitions of gender. There has been little research done on asexuality, but those studies that have been conducted suggest that, if it is a sexual orientation, it is among the least common. A commonly used description is that they feel one can get more pleasure from eating a bar of chocolate than having sexual contact of any kind with another human being or an animal.

There is continuing disagreement over whether asexuality is a legitimate sexual orientation. Some argue that it falls under the heading of hypoactive sexual disorder or sexual aversion disorder.
Among those who do not believe it to be an orientation, other suggested causes include:  

• Past sexual abuse.  
• Sexual repression (of homosexuality, heterosexuality, or bisexuality),
• Hormonal problems.
• Delayed development.
• Sublimation of sexuality for personal, religious or cultural reasons.
• Not having met the right person: some asexuals, however, argue that they do not believe in the "right person myth", because they could not get into romantic/sexual affection with anybody for long years back and do not create any image of an ideal lover for themselves.

Many self-identified asexuals, however, do not believe that such diagnoses apply to them. Others assert that because their asexuality does not cause them distress, it should not be viewed as a disorder. Those who believe that asexuality is not pathological sometimes point to the fact that similar things have been said about homosexuality and bisexuality, which are now viewed by most as legitimate orientations.
Alongside this is the debate as to whether the term 'asexual' is an accurate term. In biology, the term is used to describe a species that reproduces from a single member, whereas in this context it pertains to an organism not reproducing at all, and that 'anti-sexual' (in the clinical, rather than polemical sense) may be a more accurate description of the behavior.

There are differences among people who identify as asexual, chiefly among them the presence or absence of a sex drive or romantic attraction. Some experience only one of these, while others experience both, and still others neither. There is disagreement as to which of these configurations can genuinely be described as asexual. While a number of people believe all four variations qualify, many others believe that to be asexual, one must lack both a sex drive and romantic attraction.

The sex drive of those asexuals who have one is usually not directed at anything, and is only an urge for sexual stimulation or release; one exception is those asexuals who are also fetishists, whose sex drive is focused on the fetish object rather than a person (though many fetishists do not identify themselves as asexual). In either case, the level of sex drive can range from weak to strong, and from rare to frequent. As mentioned above, some identify as asexual yet feel sexual attraction, though some would argue that they are not asexual. Some asexuals may experience sexual feelings, but have an aversion to sex or no desire to act on them, while others seek sexual release through sexual contact.

For those asexuals who experience feelings of romantic attraction, it can be directed towards any sex or sexes. These asexuals generally desire romantic relationships (ranging from casual liaisons to marriage) but often do not want these relationships to include sexual activity. Because of their romantic orientation, some asexuals describe themselves asgay, bi, or straight asexuals; this is related to the concept of affectional orientation.

Those asexuals who do want romantic relationships are in a difficult position, as the majority of people are not asexual. Asexuals able to tolerate sex can pair up with non-asexuals, but even then their lack of attraction or desire can be psychologically distressing to their partner, making a long-term romance difficult.

Asexuals who cannot tolerate sex must compromise with their partners and have a certain amount anyway, give their partners permission to seek sex elsewhere, have sexless relationships with those few who are willing, date only other asexuals, or stay single.

Aromantic is another term for asexuals who don't experience romantic attraction or "typical romantic attraction". While some do find a relationship with another asexual, they may not include things such as kissing or touching. Aromantic asexuals who are in relationships often are unsure of themselves, even if their partner is aromantic as well, because of the mass media showing "normal" relationships. They may feel they don't measure up to their partner's standards, usually due to a past failed relationship or lack of experience. This feeling usually goes away over time. Aromantic asexual couples often look like best friends to most people due to the nature of their relationships, even though something deeper is there. It's the same deep love romantics have, but it's shown in a different way from the norm.

Some asexuals use a classification system developed (and then retired) by the founder, David Jay, of the Asexual Visibility and Education Network, one of the major asexual communities (abbreviated as AVEN). In this system, asexuals are divided into types A through D:

• Type A: has a sex drive but no romantic attraction.
• Type B: has romantic attraction but no sex drive.
• Type C; has both, but does not connect the two.
• Type D: has neither.

The categories are not meant to be entirely discrete or set in stone; one's type can change, or one can be on the border between two types. Note that AVEN itself no longer uses this system, on the basis that it is too exclusive, but a number of asexuals still feel it is a useful tool for explaining their orientation.

As a conclusion, I can say that most asexuals are physically capable of sex. Some masturbate and some don't. The distinction between sexual and asexual people is that, if asexuals think about other people during masturbation (many asexuals don't think about anything specifically sexual) it is only as fantasy. If they actually were given the opportunity to be sexual with that person there would be no attraction, or the drive would be so low as to be completely ignorable.

Historically, bisexuality has largely been free of the social stigma associated with homosexuality, prevalent even where bisexuality was the norm. In Ancient Greece pederasty was not problematic as long as the men involved eventually married and had children. In many world cultures, homosexual affairs have been quietly accepted among upper-class men of good social standing (particularly if married), and heterosexual marriage has often been used successfully as a defense against accusations of homosexuality.
On the other hand, there are bisexuals who marry or live with a heterosexual partner because they prefer the complementarity of different genders in cohabiting and co-parenting, but have felt greatly enriched by homosexual relationships alongside the marriage in both monogamous and "open" relationships.
Some in the gay and lesbian communities accuse those who self-identify as bisexual of duplicity, believing they are really homosexuals who engage in heterosexual activity merely to remain socially acceptable. They may be accused of "not doing their part" in gaining acceptance of "true" homosexuality. Some gay and lesbian people may also suspect that a self-described bisexual is merely a homosexual in the initial stage of questioning their presumed heterosexuality, and will eventually accept that they are lesbian or gay; this is expressed by a glib saying in gay culture: "Bi now, gay later."

These situations can and do take place, but do not appear to be true of the majority of self-described bisexuals. Nonetheless, bisexuals do sometimes experience lesser acceptance from gay and lesbian people, because of their declared orientation. Bisexual experimentation is also common in adolescents of every sexual orientation.

Transsexualism is a condition in which a person identifies as the gender opposite to the sex assigned to them at birth. In other words, a transsexual is a person who believes that his or her body does not reflect his or her true 'inner' gender. The transsexual has the desire to live and be accepted as a member of the opposite sex, usually accompanied by the wish to make his or her body as congruent as possible with the preferred sex through surgery and hormone treatment. In contrast, some transgender people often do not identify as being of, or desiring to be the opposite sex, but as being of or wanting to be another gender. Regardless of definition, transsexualism should not be confused with cross dressing or with the behavior of drag queens and drag kings, which can be described as transgender but not transsexual. Also, transvestic fetishism usually has little, if anything, to do with transsexualism.

Transsexualism often manifests itself as a dysphoria toward one's biological birth sex, as well as conflict living in one's social role.
If untreated, it can lead to mental and emotional problems, and sometimes suicide. It is estimated that 40% of untreated transsexual people are either institutionalized or die prematurely. However, with proper treatment, transsexual people often become fully functional members of society. The suicide statistics of treated transsexual people do not differ remarkably from non-transsexual populations.

Transsexual people almost universally prefer to be referred to by the gender pronouns and terms associated with their target gender. For example, a transsexual man is a person who was assigned the female gender at birth on the basis of his genitals, but despite that assignment identifies as a man and is transitioning or has transitioned to a male gender role and has or will have a masculine body. Transsexual people are sometimes referred to with "assigned-to-target" gender terms such as "female-to-male" for a transsexual man or "male-to-female" for a transsexual woman. These terms may be abbreviated as "M2F", "F2M", "MTF", "F to M", etc. These terms are particularly helpful in preventing confusion, because to some people the term "transsexual woman" is a female transitioning to become a male, and to others a male transitioning to become a female. When the terms transmen and transwomen are used though, it is typical for them to be used to refer to the gender that the person identifies with, regardless of their appearance or state of transition.

Transsexual people typically feel it important for people to understand that transsexualism neither depends upon, nor is related to, sexual orientation. Transsexual men and women exhibit a range of sexual orientations just as non-transsexual (some times referred to as Cis-gender) people do, and they will almost always use terms for their sexual orientation that relate to the gender with which they identify. For example, someone assigned to the male gender but who identifies as a woman, and who is attracted solely to men, will identify as heterosexual, not gay. Likewise, someone who was assigned to the female gender, identifies as a man, and prefers male partners will identify as gay, not heterosexual. Transsexual people, like other people, can be bisexual or asexual as well.

The transsexual community typically use the short form "trans", or simply "T" as a substitution for the full word "transsexual", e.g. TS, trans guy, trans dyke, T-folk, trans folk. Some may even use term that has become controversial to some, such as tranny and/or trans, despite others considering these terms to be offensive.

The most impressive hypotheses put forward to date, based upon careful clinical studies, indicate that several possible elements that cause transsexualism should be considered together: functioning of the brain and of the endocrine glands, neurological mechanisms, cultural and other environmental factors. Most, if not all, specialists in gender identity are agreed that the transsexual condition establishes itself very early, before the child is capable of elective choice in the matter, probably in the first two years of life. Others believe it is set even earlier, before birth during the fetal period. These findings indicate that the transsexual has not made a choice to be as he is, but rather that the "choice" has been made for him through many causes preceding birth and beyond his control. Advanced studies show, for instance, a difference in the human brain called the bed nucleus of the stria terminalis (BSTc), the limbic forebrain structure. In one study, male-to-female transsexuals and cis-gendered women were shown to have brains that were similar to each other in the BSTc area of the brain. On the other hand, both heterosexual and homosexual men showed male brain structuring in this area.