I hate to throw cold water on anybodies thinking but a lot of gay men have children. Biological children they had because they had sex with a lady. Until recently gay men were usually married and giving it the old college try at their marital duties. (possibly while thinking of the men they would be with later)
Or: sexuality exists on a spectrum, and many people tend towards the poles because of cultural and social pressures. For instance, everyone thinks I'm straight because I'm married to a woman. Bisexual erasure is probably a big problem in sorting out how attraction to the same sex can be passed on genetically.
Bisexual Erasure is not only a band from the eighties but how the Straight and Gay communities pretend that some people are not Straight or Gay. I think most people have mixed and complex sexuality.
I'm almost 99.9% sure that this is the answer to the question posed. homosexual people are in heterosexual relationships and producing children. One could ask, well, does that mean every homosexual offspring has at least one (1) homosexual parent? Probably not, but I'm willing to bet the number is at least statistically significant and genetics makes up the other large portion.
what if gay marriage and adoption lead to reduced genetic fitness for Gay folks? No longer a need to be in the closet so no beards no kids from the beards -> reduced allele frequency -> no more gays. Gay liberation may lead to the extinction of Gay folks.
yeah, it's ironic. If anti-homosexual people want homosexuals to go away, wouldn't it be smart to stop forcing them into heterosexual relationships and reproducing?