I had an idea last night, quite a funny one really. Now, let's get it clear, I don't beleive in being turned gay, nor do I beleive that turning straight is any more likely than "becoming less left handed". Consider populations of mammals where there is a gene for being gay (or a gene for being straight, I don't think it matters which), which in my opinion probably is the actual state of the world at present. Now, consider situations where there is no stigma attached to being gay, no fear of being caught out. In such situations, the gay mammals have no children, or at best only few because they aren't "sticking it in the right place". Now consider an environment where being gay and appearing gay is somehow frowned upon (yes, I know, the idea of being gay being frowned upon is uproarously funny). In such an environment, gay mammals may still copulate with members of the opposite sex, purely to show how straight they are.
So, religions (the source of the dominating cultural attitude towards homosexuals) have made homosexuality a taboo, a sin, and in turn have probably increased the amount of gay in the world.