Re: "Sl*tty sex"
Some possibilities with your cousin:
1. She may be needy. In between these steamy sex sessions, she may be calling the guy too much; showing up at work uninvited; and may be, in general, having a "you owe me now" kind of behavior with these guys.
2. How does she dress and behave in public? If it's provocative, well she's giving a lot away to every guy already. and she's upping the expectations of the guy who will agree to have sex with her. .......so if he was already primed to expect swallowing..... because she does a funny thing with her mouth when she's talking to a target......well, then, a facial isn't that big a leap for the guy on this occasion.
3. As most men will tell you, it's easy to get a guy in bed. So I would not even consider that an accomplishment as a step toward a long term satisfying relationship. Katie, your opening post seemed to suggest that. Your cousin may assume it as well.
4. She may also have skewed expectations. She may assume that because she can get a "10" in bed that she can get a "10" to the alter or at least in an LTR.
5. Sadly, your cousin may assume that she's being herself, being authentic, like the relationship and other self-help books tell you to be. A little bit of mystery, detachment and unavailability will go a long way in building an LTR. Not the "you can do my face" routine.