There was a lot of Infidelity songs in the 90's.TLC's "Creep", SWV "You're The One", Aaliyah's "If Your Girl Only Knew" (But not sure on this one because she doesn't sing about sleeping with the girls man, it sounds she's telling the guy in the song about what he is doing wrong).

I just wanted to clarify on that Aaliyah joint!!!
http://youtu.be/sbsBEATSkHc