Natural,
As a "nice guy" who has done some shi**y things romantically, I'll comment -- I hope this doesn't end up too self-serving, or justifying the bad stuff I did, because while it reflects my mindset, it in no way means I think it was right.
Some of us have found that when we were nice, we were either ignored by women, or used by those who didn't ignore us, without really getting anything in return (affection, sex, love, etc.)
Smart humans (and other mammals) tend to learn by example or experience, so in this case, some of us learned it was a "screw or get screwed" world, and became more opportunistic and less sensitive to the morality of our actions, since others seemed to not care when it was their turn at bat. This meant that sometimes, when something or someone came along that we wanted, we went for it, regardless of our current commitments. Sometimes we tried to justify it by saying that the person we offended against did something to deserve it, even though they probably didn't. And we did crummy things to cover up for it, unless we did finally just come clean and admit to the person that we cheated on them.
Mainly, I think nice guys who have lost out a lot earlier in their lives get very angry and frustrated about not getting the things we want, while it seems to come so easy for some guys and all women. A lot of other things in life are fairly easy or manageable to deal with (school, work, life skills, physical fitness, creativity) but the one thing we as animals are programmed for, reproduction, is for some reason especially frustrating to those of us in the "civilized world." We're fed a lot of stuff from Hollywood and the media about how our life should be at X moment in time, not realizing how unrealistic it might be. So we get mad, we get dysfunctional, we get neurotic, and issues simmer.
And then something cuts loose, and we cheat on our wife/GF/date. Try not to take it too personally. It's not you, it's us -- we're a bit broken, we didn't develop right. And it doesn't mean we don't still love you, but maybe this person we cheated on you with represents something especially significant, not even as a person, but some kind of symbol of personal achievement. We might even be partially healed after we go through this event, but of course, that's ignoring the damage it might have done to you.
That's the one downside to "nice guys" -- sometimes we sabotage the best things going for us. I should know, I've done it twice, and somehow still come out ok in the end, mainly because my wife is forgiving, and we know we're still the best thing for each other.