Generally the women Dean calls bitches aren't actually women, they're demons. Or Bela. Who, let's face it, was a bitch. An awesomely fierce bitch, but definitely a bitch. Not sleeping with him has nothing to do with it.

(inb4 misogyny etc etc I know using the word "bitch" is not ideal but jfc)

...Also I think Buffy and Dean would end up boning anyway, they have a lot in common. :B