I've never full-out cheated, but there was one time in HS that I knew my boyfriend would not be happy with the amount of time I was spending with another guy I was interested in.
I ended up dumping my boyfriend so that I could date this other guy, so technically, there wasn't any cheating involved, but a little going behind his back and sneaking around.
As for my current relationship, when we were first dating I was in a situation where I ended up kissing this girl I liked. I told my boyfriend about it immediately, we talked, and it ended up being fine. She lied to her boyfriend about it, and they ended up breaking up over it. My relationship is still going strong (about 4 years later), we're getting married in August and don't have any secrets from each other.