Being attracted or having sexual desires for the same sex is one thing. Homosexual behavior is something else. If a person engages in homosexuality that does not mean, strictly speaking, he is gay in the sense of being attracted to the same sex. If that were the case then for some just merely going to prison changes your genetically predisposed sexual orientation.
OK, I missed this earlier. A person engaging in a homosexual act is gay, as long as they're not being forced to do it. A guy fcuking another guy in prison is gay, or a bisexual, even if he only started the same sex act in prison. A straight guy won't engage in gay sex in prison - he may get raped or be made to fcuk another guy under force of threat, but if that happens, he's a rape victim, not a gay guy, because he wouldn't have engaged in that act under a free situation.
As for your other argument about gay marriages, they are simply assertions that you've based upon the bible and the view points of a couple of people, neither of which are absolutely binding, especially to people who don't agree with them or of a different faith (or not, like atheists).