I like to say that evolution isn't "survival of the fittest". Instead it is "survival of the fit enough". There is no evolutionary advantage to having an appendix, if anything the opposite is true, but it's not a serious problem often enough to get in the way of "fit enough".
Same here. A species that is entirely 100% gay would not survive, but 10% gay would not matter. As for bi, that's clearly not an evolutionary concern.
Traditionally, the best way to answer "is it genetic" (or "how much is genetic") is by studying identical twins, especially those brought up separately. That tells us, for example, that height is largely genetic (absent malnutrition of course). It would be neat for a researcher to do the work for this question. I wonder how much political pushback there would be on the study proposal, though.
