So it's not one of the more popular arguments against homosexuality, but one that I've heard is this: If everyone was a homosexual, the human race would become extinct because there would be no more offspring. Therefore, homosexuality is immoral.
Three responses come to mind to shoot this argument down:
First, it's not true that the human race would stop if everyone was gay. This is because we have the technology now to get women pregnant without sex being involved, with artificial insemination.
Second, even if everyone was gay and we didn't have the technology to reproduce with heterosexual sex, I think that if the survival of mankind depended on it, gay men and gay women would have sex with each other a small amount of times even if they didn't really like it in order to create offspring to keep the human race alive.
And third, there is an implied premise in this argument that goes something like this: If everyone being X means the human race would die out, then X is bad. But I don't see why that is such a good way to judge whether something is right or wrong. Think about this: Priests aren't allowed to have sex. So if everyone was a priest, the human race would die out. Should we then conclude that being a priest is immoral?