I've read everything so far on this thread, and have a few things to mention.
My current gf was the biggest slut I know of, and yes she's got that cat-walk of sexual condensation that drives 99% of (straight) guys up the wall. Plus she has extremely large breasts on a relatively small body frame and awesome curves which also help her get noticed by virtually everyone. And she's generally a hot personality. But enough blowing smoke up her ass, I'll continue.
She has fucked several groups and cliques of guys, and has a virtually insatiable sexual drive. She's fucked at least 14 guys (in the span of about a year), given head to around 20 guys, and been with multiple guys at once a few times. Before meeting and talking with me, she had been on a 2-month celibacy break after realizing that she was a slut and needed to exercise control. I remember on the first day of the two of us hanging out, she was almost in tears when she was discussing her revelation with me. And I saw that under her tough and thuggish exterior she was just a lonely girl with a need for validation and acceptance as something other than an object. She wanted to believe the bullshit these guys were feeding her so badly that she blindly accepted it as truth.
But we hit it off instantly, hung out with each other alone numerous times, and about a month or so later, she broke her chaste period and we had sex with each other for the first time.
It's been recently a year since that time, and we're still very much together and going strong. She and I are in love but currently in an open relationship since I'm mostly away from home and only see her once a month or every two months. But she doesn't want any other guy besides me, even though she could have any guy she wanted because they've been literally throwing themselves at her feet. She's only used the opportunity to sleep with other girls since I've left (which can have its advantages for when I return

).
My point is that some people are smart enough to understand the error of their ways, and others aren't. Your ex might just be a horny woman. But I personally have little respect for promiscuous men and women that use people just for sex. I need a connection for sex to be fulfilling.
I met a girl here who has a bf back home, who cared deeply for her. But she just wanted to get her rocks off with anyone that found her attractive (and only physically), so she was itching to cheat on him (and very callously so). I just personally find that to be pretty fucked-up. Plus she's a size queen, so she's selective. It's not the cheating that's terrible, it's the insincere nature she expressed when telling her bf about her needs. She made it as though she loved him but didn't want to be tied down. The former was blatantly false.
Casual sex is fine as long as both parties leave happy and satisfied. It's never right for either to feel used or cheap. I think that people who have given up on monogamy are just kidding themselves. No one is completely 100% secure and we all want someone to love us. If someone makes us truly happy then why not perhaps hold onto he or she?
To me, it sounds like your ex may still be a little confused in the matter.