> Your sexuality can change/grow over your lifetime, like any other part of your identity.

Actually its variability with age is very small when measured objectively. Otherwise there would exist favorable outcomes to conversion therapy. Also non-straight sexualities are less common than heterosexuality, as straight people seem to constitute 70 to 83% of the world population

I agree with what you are saying on all points, but from an evolutionary point of view there are many traits where the fitness advantage isn't as straightforward as "you make more babies". For example - the "gay uncle" hypothesis. Also, google "spandrel".

I'm in no way trying to say it's genetic but if somehow it were in some cases gays wouldn't need to have babies for it to stay in the population. Those "genes" could be present in the siblings as well and passed on. Your genetic material isn't completely lost in a population if you have close kin.

Humans evolved as a tribe/group, rather than as individuals. What does this mean for homosexuality? It means that tribes/groups containing homosexual people survived, meaning, it can still be genetic. Aside from that, it's not uncommon for gay/bisexual people to have children of their own, especially today with things like surrogates or sperm donors.