This seems to be relatively rare, but it's kind of an obsession of mine. Basically, I like the thought of having sex with another man's wife, but I don't like cuckold, NTR, or even most standard cheating scenarios because I usually find myself identifying with the guy whose partner is cheating on him, and for me, there's nothing erotic about this. However, wife/girlfriend swapping scenarios seem to provide a nice sense of equilibrium in that neither of the guys has anything over on the other, so I'm able to indulge in this fantasy without feeling that I've been one-upped. So, anyway, here's some girlfriend swapping from chapter 5 of "Ojousama wa H ga Osuki". (There's also a OVA of it as well, by the way.)