Dear Shadow,
I think you’re a clever fellow. I just see one small flaw in your post. Are you sure that the proportion of homosexual was so low as you suggested during the exponential phase of humanity demography? I guess no.
Sylas’s remark is fine. Because of the personality high diversity, homosexual behaviours are likely to appear. As far as I know, an individual becoming homosexual is randomised within a population. The social environment plays a crucial regulation role in the development of this behaviour. I don’t think genetic or black-boxed hidden mechanisms are to blame for this.
I can speak only for my self based on the logic of my personal vision, but I consider a strong relation between AIDS menace and female homosexuality. If you fancy it, this is a psychological response to harsh social condition. In term of safety, indeed.
Have a nice day,
Denesha