Gay is OK
Homosexuality causes no inherent harm, forces nothing upon anyone else, and is not associated with intentions to cause harm. The argument against homosexuality being a problem might be rephrased as "Show us why homosexuality is wrong; the burden of proof is on you."
It has never been successfully met, and yet anti-gay sympathy thrives in society. Arguments directed against homosexuality usually seem to be one of five types:
Wether it harms or offends God is not relevant; a god has just as much obligation to make a coherent argument for it's views as anyone else does. To behave otherwise would be ignoring the issue itself ("wether or not homosexuality is morally right"), and concentrating on the people involved ("so-and-so said such-and-such, so it MUST be true") - a logical fallacy.
The "holy text" which states that homosexuality is wrong - though it's claimed it was inspired by a God - would still be just as subject to human biases as anything else that gets written/copied down.
A decent god requires us to use our critical thinking skills, even on a holy text.
The holy text in question is: technically incorrect, historically incorrect, internally contradictory, hopelessly immoral, and/or irrelevant to modern society (I'm using the Bible and the Quran as inspirations here).
What god?
AIDS et al can be prevented by proper precautions,
AIDS might affect gay men more than straight couples, but it affects heterosexual couples more than lesbian ones. Does this mean lesbianism is inherently superior to heterosexuality? Of course not.
Homosexuality does not inherently require these negative attitudes.
Plenty of gay people cope admirably.
If homosexuality is associated with pedophilia and rape, then so is heterosexuality.
Even if the gay population weren't low, or even if it could spread and eventually occupy all of humanity, that would not cause our extinction: gay individuals are just as likely to want to be parents (and just as capable of being good parents) as heterosexuals. If such a situation ever faced the human race, a system of "sex for reproduction as distinct from sex for pleasure", or even artificial insemination, would be worked out.
Even if the gay population were 100%, and nobody wanted to reproduce (and it's just as possible for heterosexuals to not want children; i have to stress, this is essentially science fiction. Reproduction is not the domain of heterosexuality), then our extinction would be a "demographic" one in which individual people are not being attacked, not a violent one in which individuals may be harmed.
And, of course, if there is a genetic component, a bunch of heterosexuals would always be naturally selected for, and persist in the population.
As you can see, all the anti-homosexaulity arguments have been addressed and answered. The burden of proof simply hasn't been met, and there is no reason to consider homosexuality immoral.
Now, onto the other side: there are plenty of reasons to consider acceptance of homosexuality moral (eg// 'it's not a choice - it's just how some people are') - but society should only need one.
Personal freedom.