[rcxdude]

There certainly seems to be a genetic component to sexuality (as well as gender identity). It's not the only factor though: for example, there are epigenetic factors like the more older brothers you have from the same mother, the more likely to are to be gay.

[lazide]

Too much competition of one type makes it advantageous to flip the script eventually for a small percentage of the population, despite that having other (often heavy) costs?

Lefties have been a pretty stable part of the human population for a long time for similar reasons it appears.