Yeah like other guys here I've slept with girls who had boyfriends, some of whom I even knew casually. I was never much for attached women, but sometimes they have a way of falling in your lap. Women are sneaky as fvck, and I also don't trust other men alone with my woman.
Women love to say "well then you just dont trust her". You're right, I've been around the block a few times, and I know people are opportunists.
This can be a hard boundary to set, especially with women who have mainly guy friends. Boundaries are important, but I think proper selection trumps boundaries here. If your girl doesn't have a ton of guy friends this will be easier, and the fewer sex partners she has had the more obedient she is likely to be to you.
Every few months or so I go over my "rules" with my girl. Here they are, verbatim:
1. Don't sass me, backtalk me, or be rude to me.
2. Don't lie to me.
3. Other than group acquaintances and family, I don't want you spending time alone with other men.
4. If you are making plans on days we usually hang out, just check with me first.
Guys may laugh at this, but I have a respectful woman who tows the line. Granted, she LIKES the boundaries and leadership so it makes my job a lot easier. Her personality is submissive and accommodating. I've been in relationships with the opposite type of female----sarcastic, stubborn, contentious ball-busters---and man that was a living hell. Trying to set rules like these with a girl like that is like trying to teach a cat to sit. Poor choice from the beginning.