Obviously it's a natural process. There have always been a percentage of people born this way and there always will be. We don't understand why that is the case, it's purpose, or through what process it develops. It exists in every culture on every continent throughout time.
Social constucts dictate the level of acceptance in a given society but never drive who becomes gay. In more open societies in different time periods, gay people go about their lives relatively unfettered by others. During these times the world has not seen a significant increase in the amount of people identifying as homosexual. More may feel free to be out and open about it but their numbers aren't growing like you would expect if homosexuality were simply a societal construct.
On the flip side, when society does not support homosexuals and even persecutes them, we see that they still exist. If it were choice, why would someone risk being stoned or thrown from a roof for it? Obviously they don't. It's just who they are by nature.
That's like saying because there have always been schizophrenics, that must mean there's nothing wrong with it.
And as it happens, social constructs often DO dictate who identifies in one way or another, just as it dictates who will act on violent or criminal impulses in many cases. In a society that actually puts a certain amount of onus on the individual to be responsible for his own actions, those who can suppress urges that go against societal mores will usually do so. In a permissive society, fewer people feel any need to restrain whatever whim crosses their mind. And believe it or not, it IS possible to convince oneself that one is gay. Not saying that's the case with all of them, but it does happen.
The law of supply and demand applies to everything about human society.