Well, I don't know how they handle this situation on whatever planet you come from, but here on Earth, it's generally accepted that cheating on your significant other is a bad thing and enabling it makes you a terrible person. I'm sure, as you explore our alien culture, you will see things like "every romantic comedy ever" and "television talk shows" that heavily belabor this point.
In situations like this, which I've talked about before, I like to play a little role reversal. If you were dating a guy, and he cheated on you, how, precisely, would you feel? Because that's exactly how the wronged party is going to feel.
Or, if you're a sociopath and unable to think of other people's feelings, stop and think for a minute about what it means that this guy is going to screw around with or without you. Basically you're asking for a huge, messy emotional drama (and you're not in the right), possibly a bunch of diseases, and certainly a lot of friends viewing you with disgust. In fact, you'll be out a few friends, because they'll see you jumping right in to banging a committed guy and start to worry what'll happen if you're near their boyfriends too long.
This dude is toxic, period. Your friends are 100% right, and the only thing you should be doing is tipping off his significant other about what a jerk he is.