Homosexuality is not a choice.
There is nothing absolutely proven to be the cause of homosexuality, although many reputable people of many scientific communities agree that it is not a choice, and that it is genetic or that something happens that causes it during pregnancy or childbirth, however if the latter is true that it is naturally occurring and not a birth defect.
Studies of what causes human sexuality often become politicized, and rarely does anyone study such a thing with a neutral point of view.
Upbringing is a common argument for it being a choice, but that is impossible to prove and furthermore unlikely since every single individual's upbringing is different and there has been no absolute consistency between one homosexual person and the next. Upbringing is only proven to directly affect if an individual is open about their sexuality.
A large number of people agree that simply relying on a homosexual's experience is enough to prove that it is not a choice. Almost every homosexual says that they did not choose to be homosexual, and that they are incapable of finding the opposite sex sexually attractive. That by itself is as conclusive as any scientific study has gotten. Personal experiences of growing up homosexual are consistent with exactly what you would think they are; feeling an attraction towards the same sex in the same range of time as a heterosexual would during puberty. The largest difference between the two comes from a social standpoint, where most homosexuals know someone who is homophobic, and varying degrees of masking their sexuality become a defense mechanism.
Scientific studies suggest that there are actually a staggering number of homosexual and bisexual people who are not open about their sexuality to a single person, although heterosexuals still greatly outnumber homosexuals and bisexuals combined.