I'll put for you like this, if the entire world became homosexuals so life will end there, unlike how it been, as a hetrosexual one. It's not about culture, but about life. If your parents were two males or two females i don't think you would have posted in RF. This is not enough?
No, it's not, because it's absurd.
I'm sorry, but this argument drives me bonkers. It applies just as equally to all sorts of traits: if everyone on Earth were male, life would end. If everyone on earth were firefighters, we'd die because nobody would be left to feed us. This doesn't create any moral implications for being male or for pursuing a career in firefighting, so I don't see why it should for homosexuality.
Also, there's no Earthly reason to assume that something carried to an unreasonable extreme implies anything about that thing at more moderate levels. In high enough doses, many medications will kill a patient; does this mean that it's wrong to give them small doses that can help them?
On top of this, the argument doesn't apply just to homosexuality, but to any human trait that would preclude breeding. Is it immoral to, say, take a religious vow of chastity, or to simply decide not to have children? These things are generally socially acceptable, but they have the same implications for offspring that homosexuality would.