You absolutely can't generalize about an entire gender based on a saying. But the idea is that if someone has the personality to cheat, it probably won't happen just once. That being said, there are plenty of people who have done something stupid once, or made a mistake, and learned from it. It really depends on the person.
I cheated once, technically. The relationship was practically over, we just hadn't had a chance to talk about breaking up. In fact, when we did break up it was over the phone and comfortably mutual. We never talked about the cheating and I still don't know if he knew about it. The guy I cheated with was in another relationship himself, and even though we kept hooking up, I still considered myself single and slept with whoever I wanted to. Now, however, I'm in a good relationship and I can't even fathom ever cheating on him, it'll just never happen. In my experience, they way I felt about each of these men determined my sexual attitude toward them and my "relationship" with them, and for each guy, it was completely different.
So, it depends on a lot of things.. But mainly, how the cheater feels about you and your relationship. Would they want a second chance because they feel guilty, or because the affair didn't work out? Or do they want a second chance because they really feel like they made a horrible mistake and loves you too much to ever hurt you again?