There are occasional instances of laws that do this. For instance, Australia is one of the few countries that allows gay partners of Australian citizens to immigrate to Australia.
Except that by its wording, the law doesn't refer to gay partners, but refers to people in an "interdependent relationship" with an Australian citizen. This usually means (and was intended to usually mean) a gay partner, but it can be interpreted in a broader sense than this in the way you discuss.
All this is actually a separate category of immigration to the "spouse visa" category, which is restricted to heterosexual partners (but which, again unusually, does not require you to actually be married).
I didn't know that about Australian spousal visas--the not having to be married part, I mean. Interesting. I think that, based on their current arguments, a lot of gay marriage proponents would complain that the designation "interdependent relationship" makes us sound like lichens or volvox colonies. Well, they might not put it that way, but they'd call it "dehumanizing." It's a shame that, in the US, so much energy is being diverted into building ramshackle arguments that heterosexual and homosexual relationships have to be treated exactly the same in every finicking detail, but that's the way it is.
(BTW, the TrackBack seems to have gone through twice, even after I made a point of asking MT not to do that. You might want to delete one.)