i did once...to my 1st bf. that killed me, i was shitty at myself for doing it, and he never really let me live it down. i still wouldnt actually have a clue why i did it in the first place though, my only excuse was that it was my 1st relationship and i didnt know what i was doing (eventhough that's a dodgy excuse).
i've had 3 more bf's, and i dont think i could ever actually cheat on either of them. i couldnt live with myself if i did it again - i'd kill myself for doing it again, and he obviously wouldnt be too impressed either (even if he wasnt the jealouse type)...i think i'd be more cut at myself than he would be though, having to live with doing it once is bad enough, and i love my bf too much to even think about doing it