Why didn’t Don continue with Bethany?

Why the hell didn’t Don continue dating Bethany? I guess he started dating Faye pretty soon after. But not immediately after. Like it’s not like he stopped dating Bethany so he could start seeing Faye. And also he’s not against seeing several women at once so.

He said something like “people tell us who we are and we choose to ignore them because we want them to be who we want them to be”. Something along those lines. He narrated this right as Bethany’s going down on him.

Who is it he thought Bethany was? They talked about how they don’t know eachother and aren’t close. He said he “already knew who she was”. Did he think she was trashy for going down on him? And that’s why he stopped seeing her?

I quite liked Bethany. I think it could’ve been a good relationship to see.