"What if sleeping with some guy is squashing his motivation to go out and find the true love of his life because his immediate sexual needs are being met? Meanwhile, some other girl is doing the same with the guy you're supposed to be with."
Hook-up culture creates a strange binary: on the one hand, students are having casual sex. On the other hand, students are having no sex at all. With the exception of an occasional long-term relationship, there is virtually nothing in-between.
The problem with the hookup scene that DeeDee explains is that she's assuming it's completely one-sided -- like there are these malicious guys out there who feed off of broken hearts.