My best friend only sleeps with guys with girlfriends

So my best friend is strange she only sleeps with guys with girlfriends she says it gives her a kick and just proves how gorgeous she is that she can make any guy cheat. She is very pretty like one of those girls that just are drop dead gorgeous and can have anyone they want which is why I don’t understand why she wants guys who are already in relationships when she can have anybody else. Every time I go out with her she will ditch me for a guy she is 19 and slept with over 100 people. When we are in clubs she will ask the guys if they are single or not and if they are single she will walk away but if not she will stay and chat them up. I have a boyfriend and I won’t let her meet him I have 100% trust in my boyfriend I just know what she’s like. She is always asking to meet him which is weird. I tell her its disgusting and she needs to stop and she says no because it’s funny ruining peoples lives. Wtf could be wrong with her