Where, exactly? It seems to me like you've been arguing the opposite.
Right, and to infer from that it's natural in human society is, well, stupid.
Debatable. I would say it's unnecessary, since the fact that homosexuality is natural in human society can be inferred from the fact that it does occur naturally.
Not quite. I said it doesn't serve nature's only interest: procreation.
Nature, being unsentient, has no interest in anything, including procreation. If we want our lineages to continue, each of us individually might have an interest in procreation, but that's not the same thing.
However, if you're talking about evolutionary advantage, I'd say that it's genetic propagation, not procreation, that's favoured. It's a small distinction, but I think it's important.
That puts the ball in your court. The onus is on you to show it.
I disagree. I think that if you're going to imply that there is no evolutionary advantage associated with a trait, the burden is on you to explain why you think this is the case.
However, since plausible evolutionary explanations can be given for things ranging from a peacock's tail to pet ownership to certain forms of cancer, I don't see why your world view can't have room to acknowledge that an evolutionary explanation for homosexuality might exist.