Same-sex attraction doesn't need to have a biological/evolutionary purpose to explain its existence. The tendency to think that almost every characteristic of an organism must have evolved "for" something, or otherwise it wouldn't exist, is criticized as adaptationism.

I'm not a scientist but my working hypothesis is that same-sex attraction is an accidental offshoot of heterosexual attraction (which in is hard to argue is not an adaptation for a specific purpose) that probably doesn't serve an evolutionary purpose. There's the theory of inclusive fitness (helping relatives pass on their genes helps some your genes get passed on as well, because you share 50%, 25% etc. or your genes with your closest relatives), but I think to apply that to homosexuality is probably too "adaptationist" for my taste.

The theory of inclusive fitness sounds a bit similar to the "gay uncle" theory a couple others here brought up. It actually sounds like something between adaptationism and the "gay uncle" theory. I kind of like that. Adaptationism makes a bit of sense and seems logical, but to explain something that would otherwise not seem adaptationist (like homosexuality), inclusive fitness seems to make sense. All three theories might be able to coexist, couldn't they?

My guess would be that same-sex attraction is better explained as something developing in an atypical way, because embryo development is a complex matter where things can go "wrong" (from an evolutionary perspective; no moral judgement intended). My guess is also that something like this is behind some people "feeling like the other sex".

The idea of things going "wrong" is what's a bit depressing to think may be the case, since it seems like the original purpose of a human being becomes lost. I'd be inclined to agree with you about that, though, but in light of the theories that you and others have presented, I'm reevaluating that perspective.