By that I mean hormonal environment, as well other factors. For example youngest children are more often gay than older children. Mother's hormonal differences, as well as position in family, on top of genetic differences. The genes seem to change the odds, there exists a "predisposition", sometimes one that is very great. Witness the boys that are effeminate from childhood ... some having sex change operations, many simply coupling with men. Certainly no "choice" there, it is how they are wired.
Reading a little on paraphilias you can find some really fascinating studies into how our brains are wired for sex at an early age, more so in terms of some of the fetishes that are developed. These are not choices.