I fucked a lot of girls that had boyfriends or husbands.
Let me tell you: if a girl is flirting with a guy over Facebook and doesn't bring you up on a somewhat regular basis, she is fishing for new cock. All the girls that had boyfriends that didn't cheat would inevitably mention them. The girls that were down for dick were ones that never mentioned a boyfriend and would flirt with me like she was single and available. Well, she was available all right but often times I wouldn't know about the boyfriend until after I fucked her.
Mr. Beer Pong is going to put his ball first in a cup and then in your girlfriend's mouth.
The guys that were the "secure and trusting" boyfriend were the easiest targets. I'm not a controlling asshole but I don't like the idea of a girl just running around and flirting with guys if she is my girlfriend. For one, it gives guys the wrong signals and unless she has an IQ of 12, she knows this as well. Next, it leads to unfortunate circumstances. The guy steps it up and she steps it up. Soon, she finds she actually likes the guy. In the movies, it is always the vindictive asshole whose girlfriend cheats while the secure guy never has that problem. In reality, the "secure guy" is the one who is getting played.