Many men separate love and sex. This is especially true if what they mean by love is on par with "caring a lot about you" and not being "in love" with you.
In this scenario there is a double betrayal. You could have asked:
What kind of (friend) has sex with your man behind your back?
What kind of woman considers staying with a man who cheats?
Ultimately it comes down to what (you) are willing to put up with. Each of us (chooses) our own friends, lovers, and spouse.
Each of us is entitled to have our own "deal breakers". You have to decide is (cheating) a "deal breaker" for you. What (he) wants or expects you to be "okay" with is irrelevant. You get to (choose) how to respond to this situation. The ball is in your court.
It's a waste of your time and energy trying to "figure (him) out". That's a job for a therapist. You have to determine what (you) want to do.
There are only two ways to experience joy and peace of mind in relationships: We either get what we want or we learn to be happy with what we have. Stay or move on. The choice is up to us.
"Never love anyone who treats you like you're ordinary."
- Oscar Wilde
Clearly if someone is having sex with your friends behind your back they don't think (you) are all that "special".