I feel like some guys really do learn their lesson..
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I'd say it depends, although I do feel that that statement is mostly true. I think it takes a certain personality type to even allow that to happen. I know just as many people who couldn't even come close to cheating. They could have a naked victoria secret model in their lap and they'd still tell her to leave.
Looking at the other side of the coin, I also know that just about every person I've ever known who has cheated, has done it more than once. Many times these people will also feel guilty about it. Most people who cheat are well aware its bad and will feel some level of guilt. Only a select few are just so cold that they just don't care.
That said, on very slim occasions mistakes do happen and its up to you to analyze all the facts about what happened to see if it was something that was largely out of their control. For example, one of my friends who has a history of cheating, is by all intense and purposes a good guy, with good intentions, and a good boyfriend. He is generally loyal to his GF's and treats them very well. His current Girlfriend whom he's been dating for some 4 yrs, he cheated on a few years back. His roommates were having a party and he got blackout drunk (truthfully his first mistake). There was a girl there who was known to be a bit easy and would try and find ways to weasle her way into bed with guys (she wasn't attractive so she had to manipulate her way into the bedroom). Many of the stories I heard was that she basically came onto him and he was so drunk he didn't even know what was going on until the next morning when it all sunk in. She had basically snuck into his bed while he was passed out and pretty much got him to have sex while he was still in a drunken stupor. While I still think there were precautions he could have taken to avoid such a situation, and regardless of how truthful his story and others stories were, I do truly believe that after taking to him, he was genuinely guilty about what had happened and years later still loves his Girlfriend dearly.
It's tough, especially since only you truly know your boyfriend and the situation. I personally have a pretty low tolerance for cheating so it be pretty hard to stay with me after cheating regardless of the circumstance, but I do feel there are some very very rare situations in which a pass should be granted.