I smell a whiff of jealousy in your judgment of Cassie. I could be wrong, but I don't think so.

Cassie had a lot of dates. Cassie had a lot more opportunities to see if there was an emotional and/or intellectual connection to guys than you did.

Just because Cassie flirted didn't mean she slept with all of them (or any of them). And if she did, then she got to know if they were sexually compatible (at least at a basic level). Sexual compatibility can be just as important to a relationship as sharing values, depending on the people involved.