There is no "gay gene" just as much as there is no "black gene" or "smart gene". Genetics is a complicated field where several small changes within the human genome can result in absolutely no difference, or a disastrous mutation.

Also, if it's impossible to base something's occurrence on biology because of a lack of genetic information, then the entire biology field is null and void. You're using very juvenile logic of you need to understand everything about a system to understand a small part of it. A child can learn basic addition without knowing calculus; a scientist can determine the extent of biological or environmental correlation without understanding the genetics. A very simple one would be identical twins. Identical twins are more likely than fraternal twins to share the same sexuality, who in turn are more likely than related siblings to share the same sexuality. Even when raised in separate environments, identical twins share their sexuality just as frequently than if they were raised together. The evidence to support a strong biological influence has been coming out steadily for a while now.

And using your logic, more individuals would have been homosexual in ancient times, as consensual homosexual relationships were not only common, but encouraged among males for the sake of developing "brotherhood". However, history shows that humanity not only survived through the ancient era, but didn't even have a decrease in population attributable to a lack of mating.

In addition, males will get an erection from having their prostate massaged. It's an organ that is not reachable except by sticking something into a male's anus. Using your logic, all males who receive a prostate exam are at risk of "contracting" homosexuality. Using your logic, people who masturbate extensively will become homosexual because they achieve pleasure through somebody of their own gender. The environment has routinely been proven to have little correlation with instances of homosexuality. You assume it does though, and fail to bring up proof to defend it. No matter how weak my evidence is, it's stronger than your base-less assumptions.

Homosexual relationships are documented in animal species. Look it up. There are animals that go their whole lives without mating with the other gender, and instead have a steady partner with another animal of the same gender.