I think it is not a choice. People born that way. The fact that many gay men have tried to change and have not been able to is suggest that this is in-born and not an environmental or psychological cause. Scientists used to believe that homosexuality comes with genetics, but the failure to find such a gay gene is dimming the possibility of this. Now, most scientists are fairly certain that homosexuality is neither environmental nor genetic, but our sexual orientation - whether you are gay/bi/straight - is decided while we are in the mother's womb. So homosexuality is mainly suspected to be due to too much or too less amounts of hormones introduced to the fetus in the mother's womb. By the way, remember, homosexuality is not seen only among humans. Plenty of animals exhibit homosexuality/ bisexuality too. Its not an unnatural thing invented by humans.
Most organizations who claim to change sexual orientation are Christian Evangelic organizations that do so because homosexuality is condemned in the bible. This is very dangerous because they lead their clients to believe that
1) They can change their homosexuality
2) God will send you to hell if you're gay.
The problem is, when a person goes to these people for help, and tries to change themselves, can find that they cannot change, they end up hating themselves. There are many cases in the U.S. of gay men committing suicide after seeking help from such organizations.
One trick these psychologists try to do is try to make other areas of your life interesting so that you don't think about sex much. So most of the people who claim they have been turned heterosexual after being homosexual are people who live celibate lifestyles. There have never been successfully proven cases of making a gay man interested in women.
