I have not, and I would never. I've been cheated on before, it's an immature behavior and a shitty situation to be on the receiving end of. If one is really so discontent with their relationship that they would seek out their intimate needs with someone else, have the stones to either bring up the problems in the relationship and try to fix them, or end the relationship before one seeks out someone new. It sucks to break a heart, but it sucks even more to betray it.