Fraternal birth order effect is a thing. Which is something that does happen before birth technically, but isn't strictly genetic.
Funny enough, it also demonstrates that there are have to be many different biological routes to the same forms of atypical sexual tendencies.
Add to that the well observed ability of traumatic events in childhood,
especially sexual abuse, to correlate with development of many types of unusual sexual behaviors
both more and less illegal, and here you have a psychological, post-birth route to it too. Not to mention that we even have historical and current society wide cases with certain odd cultures, like ancient Greece and modern Afghanistan, that have an unusual numbers of men attracted to boys.
Between the "abuse effect" and the cultural differences, i think it's established that it is possible to manipulate the sexuality of a person after birth, even though the exact mechanisms aren't well explored, their reliability is hard to quantify, and researching it further is both politically and ethically challenging... though it has to be done as early as possible for effectiveness, early teens at the latest, which does raise certain alarm bells with initiatives like early childhood sexual education and drag queen story hour, while also having implications for discussions around related topics, like conversion therapy generally not working (generally too late), and the leftist meme of "if being gay is a choice then choose to be gay to prove it" (it's not exactly choice to be abused and you're probably too old).
And then there are the more and less famous cases of adult men, sometimes pretty old and normal, with a wife and children, suddenly deciding that they want to be a woman, which is even beyond the above pattern.