I'd say cheating (yeah I used the word, sue me) creates more problem than it prevents - at least if you get caught.Of course, to be honest is something that I mean "by default". I can do whatever I want and a girl can also do whatever she wants. It prevents relations from a lot of problems, stupid quarrels, suspicion etc.
Personally I would feel very hurt if a girlfriend of mine was sleeping with someone else. No matter what the reason was or what she said to defend it, deep down it would feel like a betrayal. And that's something I would never want a close one to feel, so I'd never do it myself either.
Maybe I'm sentimental or old-fashioned, but that's my 2 cents.
We're all horny monkeys, but if you can't hold that back - don't get involved in a relationship. Or even better, find a girlfriend who'd be okay with a threesome. :thumbsup: