It can't be nature. If it was in one's genetics, he or she wouldn't be able to reproduce; therefore, his or her genes that determine him or her becoming gay would be destroyed.
I suppose that it could be hiding in genes, and just have a very low chance of becoming dominant. But if that were true I doubt we'd see as many homosexual people as we do.
BUT, after having said that, here is a perfectly reasonable explanation as to how homosexuality could be due to our genes.
Maybe homosexuality is natures way of keeping population low. Perhaps we go through cycles, and every one hundred years or so, the vast majority of the population becomes gay and stops reproducing. The population becomes low and those homosexuality genes die off; which is due to the gay population(who were the carriers of the gay gene) being unable to reproduce. Then in a few hundred years, the gay genes pop back up(as there would be gay genes in the heterosexual population also, just not dominant; therefore that gene would not die off completely).
Of course, thats all theoretical; however it sounds completely possible to me.
I suspect we'll find an answer to this question when we learn more about the genome. No one can really answer this question without more information.