In a word: bitterness.
No matter who you are, and the cheatee becoming the cheater isn't limited to guys,being cheated on hurts. And it's not the kind of emotional hurt that you get over easily. That kind of pain tends to change some aspects of your personality, not least giving you a nasty dose of trust issues.
The basic thought is "I'm going to get cheated on, so I'll beat him or her to it!" It's not smart, or logical, but that's the process. Which is why I call out cheaters harshly on here: the damage they cause tends to radiate well beyond just one person.
It's tough to be with someone who has been cheated on. You constantly feel you have to prove yourself to him that you're not like them cheating b*tches. It's hard for him to be emotionally open AND emotionally close to you which really sucks because this is very important in a relationship. You know he's desperately trying, but still, as cheesy as this may sound, it's like having to pick the pieces of his broken heart which you weren't even responsible for to make him whole again. And while you're doing that, you still question if he'll stay long enough to realize what you've been trying to make him see: you are NOT a cheater and you LOVE him despite the fact that he's been cheated on and regardless of a very likely truth that the cheating happened partly because of his own inability to fulfill his part in the relationship.
I agree with everything but the last part. It's NEVER the cheated's fault. If they are unable to fulfill their part in the relationship, then the partner is entitled to break up with them, but not to cheat on them. Sure, it could be their fault the relationship was failing, but a person decides to cheat on their own through pure selfishness and disregard for other people. People cheat because there's a break-up worthy problem like that and they don't want/care enough to accept it or put forth the effort to fix it. Or, they're compulsive cheaters and they can't help it. The latter is especially not the cheated's fault.
I speak from experience being with someone who has been cheated on thrice out of 5 past relationships. I now see why those women chose to still cheat (absence), but heck, I'm not going to be like them.
Its not necessarily bitterness, the guy or girl (girls do the same thing) learn to accept that behavior and starts to exhibit it. Its kind of like if a person were molested as a child, they will likely end up molesting kids as an adult we well (extreme example). AGAIN, Im talking about it likely to happen, not that it will. Some guys/girls will learn to cheat after getting cheated on, but thats not always the case. Some may even learn to harbour hate for the other sex and have insecurities. It all depends.
I was cheated on by my last boyfriend- and he did it with my best friend. Needless to say, I was pretty f-ed up after that and was a complete mess when it came to being in a relationship again. I'd feel like I was losing my mind (and control of the relationship, which I felt I needed) when I'd see my new bf talking to another girl or mention another girl.... I just couldn't deal. I knew I was overreacting but at the same time, I couldn't stop myself even though what he was doing was harmless and in no was a threat to our relationship. It got to the point where I convinced myself he would cheat and so I all but beat him to the punch- which I completely regret now because I finally understand that he isn't an asshole looking to gain my trust and throw it back in my face. He never found out what I did but I feel horrible about it every day. I am trying to move on and learn from my mistakes, but I now I fully understand how damaging cheating is, even after the original transgression is long over.