I've never cheated, but as I've wrote at some length about on the forum, I've been the other guy...many times. That's something I've given up though.
Rationale:
By one standard, I don't have any commitment to whoever the girl is dating or seeing. If it's consensual between me and her, well, that's fine. I can't really rule it out on that basis. There are however, the real implications of messing up their relationship and hurting them and potentially, if there are children, messing with them.
But what it really comes down to is this: I don't want to be involved with dishonest people. If you don't have the ability to be straight with someone who is apparently your 'significant other', then you're probably dishonest nearly everywhere else in life. Individuals who lie about these things lack backbone and need to step up and manage their insecurities. They are part of the "big picture" problem of a society which cannot or will not communicate directly with each other in order to foster the change which is needed to keep things running smoothly. These aren't people I want to be involved with, and it's not someone who I would like to be.
Since I've "gone good" I have gained a lot of pride from my honesty. I tend to be very upfront with everyone I get involved with. I really regret messing around with all these girls. It's brought me little but short term thrills followed by longer term angst.