I can understand why you might feel the way you do with regard to him being with another woman but if you look at it from his side, he's probably just wanting to have a drink with his mate...who happens to be female.
I guess the questions to be asked are:
1 - you don't want them spending time together - do you think your bf has more-than-mates feelings for this lass and is going to crack onto her?
2 - do you feel threatened by her?
If yes to either of these then you have to address your relationship and trust within it. However, from what you've said about purposely bumping into his car, it sounds like you're very insecure and perhaps emotionally not ready for a relationship. Ultimately, if this does upset you and you can't move forward from it, then you shouldn't be in with him because you will only lead to arguments which won't be nice for either of you.
In answer to your Q, I really do think you're being unreasonable; if he'd only just met this lass OR he had a sexual history with her then I'd agree with you but he doesn't and if I were him, I wouldn't go back out with you.
Sorry if it sounds harsh but these two people are friends and you don't have the right dictate which friends he does and doesn't spend time with. How would you feel if he tried to stop you seeing your best friend?