Today, the sexual orientation of transgender people match the gender. A trans woman who experiences attraction only to women is a lesbian. A trans man who experiences attraction only to women is a straight man.
Sexual orientation sees a more complex understanding of both sex (male, female, or intersex) and gender (man, woman, or third gender).
A gay or lesbian person may do sexual behaviors with someone of a different gender for many reasons. One reason is the desire for a family that looks common. Another reason is to skirt bias. A third reason is to skirt punishment in religion. Not all LGBT people keep quiet about orientation to a wife or husband. Some who come out as gay or lesbian have a healthy continuous heterosexual marriage.
Sexual identity can change through the life of an individual. This may or may not line up with sex, sexual behavior, or sexual orientation. Sexual orientation is stable and will not change for most people. Still, some tests say that some people may experience change in sexual orientation. This is more common for a woman than for a man.
Scientists do not yet know the exact cause of sexual orientation. People thought that homosexuality was the result of wrong brain development. Early life experience and sexual abuse was thought to cause wrong brain development. This thought process comes from bias about homosexuality. Now many people guess that biology and environment play a complex part as a cause.
Tests link many biological factors which may have relation to the development of sexual orientation. The factors are genes, prenatal hormones, and brain structure. Tests are continuous in this area.
Scientists have a belief that sexual orientation is not from any one factor. Instead, this is thought to be from a combination of genetics, hormones, and environment. There is more proof that supports a biological cause of sexual orientation than a social one. It is not thought that sexual orientation is a choice. Some scientists have the belief that this is set at fertilization. Statistics say there are biological differences between gay people and heterosexuals. This may result from the same cause as sexual orientation itself.
Genetic factors
Identical twins are more likely to have the same sexual orientation as fraternal twins. This means genes are linked to the development of sexual orientation. Studies have not found single 'gay genes', instead several thousand genes of a small influence are involved in sexual orientation. There are issues with the methods used in this research.
Hormones
The hormone theory of sexuality holds that certain hormones play a part in the sexual orientation that shows later in a mature person. Hormones in the fetus may be the prime effect on sexual orientation. This also may be one of many factors that combines with genes, environment, and social conditions to make a sexual orientation.
In humans, it is normal for females to have two X chromosomes. For males, it is normal to have one X chromosome and one Y chromosome. A human fetus starts as female so the Y chromosome is what makes the changes necessary to switch to male. Androgen hormones control this process. These androgens are testosterone and dihydrotestosterone (DHT). The testicles in the fetus are responsible for the existence of androgens. The androgens will work together to make the sex difference of the fetus. This results in differences between males and females. Some scientists change androgen levels in mammals during fetus and early life as experiments.
Birth Order
More than one study says the chance of homosexuality in a male increases with each successful birth of a male child. This effect ends if if a male uses his left hand more than his right hand since birth.
"Fraternal birth order" (FBO) effect is a theory that has strong proof of its prenatal origin. No proof so far links this to an exact prenatal process. Tests give the idea that this may be from the immune system. This is a immune reaction from the mother against a substance important to the development of the male fetus. This reaction is even more probable after every successful male birth. It is thought that this immune effect changes future males prenatal development. This process is the maternal immunization hypothesis (MIH). This starts when cells from a male fetus enter the circulation of the mother. The immune system of the mother cannot support the Y proteins because she is female. This causes the development of antibodies. An antibody travels through the placenta into the section with the fetus. From here, an antibody crosses the blood–brain barrier of the brain of the fetus. This alters sex differences in brain structures relative to sexual orientation. This cause the son to experience more attraction to a man than a woman.
There is little proof to support the suggestion that early life experience or sexual abuse play a part in connection to sexual orientation. The viewpoints of parents may have an effect on how a child experiments with behaviors that link to a certain sexual orientation.
"See also: Conversion therapy"
A sexual orientation change effort is a plan that will try to change a same gender sexual orientation. This is cognitive behavioral therapy or conversion therapy.
No mental health organization gives approval to any effort to change sexual orientation. Almost all have a statement that says not to get a treatment that claims to change sexual orientation. Some orginizations with that statement are the American Psychiatric Association, American Psychological Association, American Counseling Association, National Association of Social Workers, Royal College of Psychiatrists, and Australian Psychological Society.
In 2012, the Pan American Health Organization makes a statement that says not to get a service that claims to change people with a not heterosexual sexual orientation. This is because there is no medical need. This is also a serious threat to the well-being of people that use this service. The scientific consensus of the world is that homosexuality is a normal and natural form of human sexuality. Homosexuality cannot be thought of as a disease. The Pan American Health Organization also says governments, schools, organizations, and communication outlets need to support respect for differences. Sometimes, a gay child will have to go to a "treatment" without choice. A gay child is sometimes put in a place separate from everyone for many months. Any places that have such poor treatment be subject to punishment under government law. This is for two reasons. The first reason is because this is against the moral concepts of medical care. The second reason is because this is against human rights that the international agreement keeps safe.
The National Association for Research & Therapy of Homosexuality (NARTH) takes takes issue with how the mental health society describes the success of conversion therapy. NARTH also takes issue with how the mental health society describes sexual orientation for two reasons. One reason is because it is not a fixed two part quality. The second reason is because it is not a disease. Instead, it describes as a continuous order of sexual attraction and emotional attraction. The American Psychological Association and the Royal College of Psychiatrists say there is trouble with the positions of the NARTH. Science cannot support the positions. The positions also make an environment in which bias increases.
Different definitions and strong social rules about sexuality makes sexual orientation hard to measure.
In the 1860s, Karl Henrich Ulrichs makes small books about a sexual orientation organization system. The organization system is only for males. It separates males into three simple groups. The groups are "dionings, urnings" and "uranodionings". Ulrichs outlines a set of questions to check if a man was an "urning".