Does there exist/do you have an evolutionary explanation for homosexual behaviour? Please read before jumping to conclusions.

TL;DR - How has (at least exclusive) homosexuality survived natural selection, when it means genes can not be passed on (and thus, the predisposition to be homosexual in the first place should have died off thousands of years ago) HAPPY NOW MAGNON?!

As an armchair psychologist (only have an A Level in it), and a mostly evolutionary one at that, I always found the topic of homosexuality fascinating. Now, before I say anything else that may lead to confusion and downvotes, I want to clarify that I have no issue whatsoever with someone who fancies their own sex. So long as you are not hurting anyone (against their will that is wink wink), or arguably yourself (although I suppose you have a right to do with your own body what you will), then you can do absolutely anything you want. I have gay friends, who are awesome btw, and a couple of managers at work are happy and married lesbians. In fact, I have a lot of respect for people who are willing to fight against oppression and stigma in the name of freedom and happiness. If this submission offends anyone in any way, it is not my intention and I think the chances are that you are misunderstanding my question or motives.

That said, from an entirely unbiased, scientific and evolutionary perspective, homosexuality (of the exclusive kind, that is) is a dead end. You can not pass on your genes. Obviously nowadays, thanks to science, there are all sorts of things that can be done, if passing on your seed is important to you.

However, back in the day of cavemen and hunter/forager behaviour, homosexuality would have ended; it has no fitness advantage, largely because there is nothing you can be passing any advantage on to. By all accounts, any genetic predisposition to exclusive homosexuality would have been thrown from the gene pool.

So that poses two possibilities;

A) Any genes carrying a predisposition to homosexuality also have a benefit to them (applies to non-exlusive homosexual behaviour); this keeps the genetics in the population through enhanced fitness.

B) Any genes related to homosexual behaviour are/can be recessive, requiring a mixture of genes or circumstances to be activated. (This overlaps with A somewhat)

C) Homosexuality has no genetic basis whatsoever, and is an entirely learned behaviour (whether it be through childhood experiences or anything else).

Personally, I don't see C as a general rule at all (not that it can't happen, I would imagine all kinds of things can make you psychologically, but not biologically, gay), and I think it is offensive and immoral to assume that anyone engaging in homosexual behaviour is only doing it for any reason other than that is simply their sexual orientation.

However, that still leaves me stumped for an explanation that covers all the bases. The fact that such behaviour is widespread across the animal kingdom suggests to me that there has got to be something to it that promotes fitness. But I am not nearly educated enough in biology nor psychology to come up with a decent explanation.

So please, reddit, help me out here.

EDIT: Added a TL;DR