I agree with Jeff - cheating is lying. If you're lying to your SO, THAT'S cheating, but if you an open relationship and have sexual relationships with other, I don't consider that cheating. For instance, Bill probably shared details on his experiences with Hillary.
IMHO, someone that actually cheats on their SO is a coward - they're not brave enough to be loyal, and they're not brave enough to leave the relationship so they can play.
