What is Casual?

So I have be hooking up with this guy and we have had sex for more times than I can count, more than I have had with anyone else. Not that he is so good at it, but because I just feel comfortable with him, I have noticed a few things from the first time we started he has gotten better and better and I don't mind that he has a girlfriend, but I a have a few questions.  so we got to play a game of truth and dare the other day and he got to mention he had sex with my before best friend, that is now my arch enemy and not did he just hook up with her but in the Same 24hour we had sex.
I like the constant sex, having someone there you can just call on to get it off..but when is Casual no more casual? Because I feel insane when we are having sex and at that point I just want him to be mine and mine alone.
Should I cut all connection with him?
Should I feel disrespected that he slept with her.
When is the time to draw the line?
Why do I feel betrayed?