Don't get me wrong, same-sex-relationships occur naturally amongst most (or, heck, even all) species of mammals including humans, but forming and declaring stable relationships is a matter of culture. I.e. in Muslim tradition it's not that uncommon that two men engage in... erm... cuddling, but they won't talk about this, won't think of it as "homosexual acts" and won't refer to it as something more big than just "friendship" (or if it is about "love", than nothing bigger than love for just friends or children). Another good (and compared to today's standards rather strange) example is pre-Christian Viking culture (including punishment through anal raep etc.)... or the (same comparison) very tolerant Greco-Roman or ancient Egyptian cultures.
There're far more "homosexual" acts within the "heterosexual" population than mainstream media would ever even dare to suggest - it's just fear to both break the cultural taboo and depart from common pseudo-scientific convention.
Fuck yeah, sociology!
DF doesn't care about any of those things... in fact there's just fertilization through pollination (or telepath sex, whatever) and both males and females rip each other's loincloths off at sparring, even in public. The only things discriminating are that
- only females get pregnant and
- only mixed-sex-couples will marry or call themselves "lovers".
And I'm quite sure even this will change when the RAWs are completely in function (not necessarily in vanilla DF, but it's a game from the US so who cares).