Not entirely true. While we (those of us involved in the scientific study of human sexuality) don't know the exact amount of variance accounted for by genetic endowment and environmental influence, we do know (at least with men) that gay affectional orientation is no more a choice than heterosexual affectional orientation. I didn't choose to be straight, I "discovered' it.
One might use the analogy of left handedness. We know that a certain percentage of the population is left handed, and expectable variation in later dominance. So, too, is gay affectional orientation...and expected variation (NOT deviation) of affectional orientation.
It is also the case that affectional orientation are very plastic...not necessarily set in stone for life. But that is a very involved discussion for another time.
Suffice it to say that there is an enormous field of scientific study involved in studying the basis of affectional orientation, and although we are far from certain of the "cause" of heterosexuality, or gay affectional preference, we are not totally ignorant on the subject, either