Hrm....I'm not a guy but I'd like to explain my theory... here goes:
When a guy is in his teens, he is all about fitting in with his buddies. He wants to outdo whatever anyone else does. His ego is looking for a boost. So, in this case, he might want to be able to say to his friends, "Hey, guess what. I messed around with "Jane" earlier today and then I was hooking up with "Lucy" later on at a party!!" It's like he wants to see how many chicks he can get with. He knows that his g/f will be there if he needs some play so it's kinda like a security blanket. But he may find some hot chick at a party and "forget" (purposely) that he has a g/f and mess around with that chick. This coould turn into a chain reaction...he breaks up with his g/f, dates the chick from the party...cheats on her with another chick...and so on...
Now I don't know if this makes sense to you or not but this is my thoughts on cheating. Guys, am I right, or what!?
