Sexuality in humans is part social stratification and part impulse... Who you have the opportunity to mate with isn't up to you. That's why I don't believe in identity politics. Most of us will learn who is a mating option and favor that over others because we have learned how to woo that particular sex. There is no such thing as biologically gay, it's just nonsense. Most gay people don't pick to be as such they are rejected by women and learn to succeed with men. Once you receive progress along one path of sexuality you will continue along but it doesn't mean it's fixed. I've got "gay" (self-identified) friends who dated women and men, but were wanting the lifestyle of being gay over being straight.
We are humans and need love, sexual stimulation, and communication and we will seek that via whatever avenues we are successful at. If our only option for this positive feedback is the same sex we will seek it, if we can seek it with both we will, and if we are more successful with the opposite sex we'll go there too. This is largely why I think discriminating on sexual orientation is garbage, because our apparent orientation can change at will if we ignore the social ramifications of our peers and their expectations. The rules and the strata are basically worthless garbage for people to self-identify and discriminate between each other with. We will seek our love with whomever we resonate with regardless of these particulars.
Perhaps, but if they do eventually find some genes involved to definitively say some are just born that way - a possibility at least - then the question will surely be answered (for them, not discounting the other options though).