There shouldn't be any racial or gender restrictions in my opinion.
How come the NPC can not have a personal preference to what they think is attractive be it a specific gender or race? It wouldn't make sense if every single person in Skyrim would marry every imaginable race/six combination you could play. If this were a six game, sure. Since it's not, it would make more sense that NPCs have varying restrictions to what they are attracted to. Everybody's got them to one degree or another.
I wouldn't expect much depth because if you think about it there isn't much depth in stuff like advancing through guild ranks either.
I expect depth though because a relationship is more personal and involved than hanging out with your buddies in the guild. It's different from any other type of relationship so it would be cool if it did have a complexity and depthness to it.