Well, I like and dislike that idea at same time.
At first, I don't think Toady will ever add non-heterosexual relationships (and poo, urine) even when he will move on relations&sex rewrite.
But that could be implemented in likeness to specific person based on someone's tastes, like Urist likes Meng for his golden hair, because Urist likes golden color. If Meng and Urist are male dwarves, and Urist find Meng absolutetly perfect by his tastes and he's not somewhat homophobic&he doesn't care about authority much, successfully embrasing civ's ethics, that could lead to interesting results: Urist likes Meng, but Meng hates homosexuals and finds Urist's winks and flowers (mushrooms/gems/etc) absolutly disgusting, so Urist accumulates negative thoughts from being rejected and goes into tanthrum, starting the fall of fortress. What a tragedy.
That's much more deeper than just homo|hetero boolean logic.
That also requires adding special ethics tag, that will judge civ's views on... perversion? (I don't think that homosexuality is normal, that's my personal view). Like, goblins (or elves) will slightly care about that, but humans won't tolerate perverts and will burn them to crisp.
Gay fortress? Oh no, please, let us disable that in d_init file.
So, I don't think that feature should add only homosexual relationships, that's just too... plain somewhat? (Don't know how to name that, sorry for my English, I'm not a native speaker) Other forms of non-standart sexual behavior (define standart sexual behavior using ethics) should be integrated with this (like, Urist likes tigers for their stripped hide and so he's attracted to that tame female tigerman, but that conflicts with his civ's ethic view on non-same race sex).
After all, it's a very major overhaul of relationships and I doubt that Toady will do that just to add gay dwarves for your pleasure.