My point was about being clear in what you want, both with yourself and with the girl. If you genuinely want to be in a monogamous relationship, you've got to go by the rules of one. If you get into a monogamous relationship and don't have the willpower to stick to it, you should get out of it, or at least talk with your partner about it before you go off trying to fuck other people. You described yourself as being open to having sex with other girls behind your partner's back. Frankly that's cowardly, and yes it makes you a mug.

My situation is more common than you'd think. I sleep with other girls they're free to sleep with other guys. Happens a lot. The way you get into that situation is by being open, honest and clear about what the situation is right from the start. If you want to shag a bunch of girls, I suggest you make it clear that it's just sex. Provided you're cool about it a lot of girls are happy with that. Girls enjoy casual sex. Who'd have thought.

This again all comes down to trust, honesty and consent. If a girl you are in a monogamous relationship with has no idea you might be shagging someone else and would not be fine with it then going out and shagging other girls is not fucking fine. It's cheating. Shame on you if you do that, and I feel bad for any girl you do that too. On the other hand if you want to be in a polygamous relationship, you discuss it with her and she's fine with it, great. Have fun. But she'll get to have her fun too, so you'll have to deal with that.

Figure out what you want and stick to it. Don't lie. Don't cheat. Nothing good will ever come of it.