Selina is both moronsexual and attracted to high intelligence and bruce is the only person she knows who hits both of these so hard.
Bruce: *figures out the riddlers latest scheme is tied to a cold case and solves it to stop him*
Selina: Oh I’m gonna sleep with that man.
Bruce: *buys one of every brand of cold medicine because he doesn’t know which one jason prefers*
Selina: Correction, I’m gonna marry him.