We are just replicating DNA just like all other organisms on the planet. But, a beaver living in over crowded conditions, will eat pine bark to abort a pregnancy so as not to make the situation worse. This is not a conscious choice, it is instinct, something that seems to be built into most species. The human race has never lived in such huge crowded cities as we have today in all the rest of its time on this planet. Cities, like Toronto, and Montreal, are, for all intents and purposes, unnaturally overcrowded. Perhaps male/female sexual preferences are the norm in nature, but cities, being as they are, tend to produce humans with a variety of sexual preferences possibly in an attempt to control the over population. In a natural, open setting you may not see so many infertile couples as you do in the city.
Any way, I don't think that under the conditions of city life, homosexuality is any more unnatural than living in such overcrowding. I think nature and our DNA have built in mechanisms to balance these situations, so I don't think sexual orientation is a choice, but has a lot to do with environmental conditions.