From the thread title, I'll assume we're talking more about the legal aspect of homosexuality. Of course, one could generalize the different arguments. But questions like "Is it wrong?", "Is it a sin?" etc, are complicated and create more fuss, and less social impact.
-I see that you're going all over genes vs choice. In my opinion, that dilemma is totally irrelevant. Suppose being gay is 100% dependent on choice (as I believe it is). If someone is about to do something homosexual that has no impact on any other person, is there any good reason why they shouldn't do it? Seemingly no. But if there is, it would obviously be applicable to the case where sexual orientation is well defined from birth.
-Yes, it seems that there are frequent occurrences of homosexuality on other species, but that's also irrelevant. We are human. If some of you are not, then the laws are not applicable to you anyway.
-Free speech. Homophobia and using "gay" as "lame" on one hand, Hollywood, media and subliminal messages on the other. You can't tell people and corporations what to say and do. After all, it's naive to want to be liked by everyone.
-On "cultural effects", like how some people believe that homosexuals act indecent and, as such, create a degenerate image of one's nation, you might see that those "homosexual indecent acts" are just different versions of "heterosexual indecent acts". For example, if you're a guy:
Insistingly hitting on a guy <=> insistingly hitting on a girl
Having multiple gay partners <=> having multiple girlfriends
Revealing your gay deeds on others <=> bragging about your multiple jackhammer adventures
Dressing, talking, shaking like a dumb little girl everywhere you go <=> Dressing, shouting, standing like a total douchebag everywhere people tolerate you
Watching gay porn <=> watching other types of porn
Imposing your sexuality <=> imposing your sexuality
etc.
However, it is possible that there are exceptions. Do they have a reasonable impact?
-Perhaps I missed the related discussion, but health implications are the real issue. Things like HIV and anal sex. One should not only think if these implications are real, but also if the role of the state is such as to regulate such things.
-And finally, gay rights. It's an issue of the economy, really. More government spending available = more social offerings. A few years ago, gay rights briefly emerged on the Greek media. Now it's about people-who-barely-make-it-anymore rights. I believe a marriage can't offer anything to a couple in an emotional perspective, unless it's a couple that wants to bring personal affairs to the public (which is bad for love). To be clear, I believe that being homosexual relationships should be legal, as long as there is consent.
I hope it wasn't too long.