I'm not going to accept my best friend's girlfriend.
Why aren't you going to accept her?
Because I don't like her.
It's fine for you to not like your best friend's girlfriend, but you should try to accept her. It's not right to be rude to people that your friends care about. If you're rude to her then your friend might not want to hang out with you.
Okay fine, I'll potentially save myself a headache.
Who knows, maybe you'll get to like her if you get to know her! Also you'll be saving your friendship.