While she's still your 'girlfriend', and you don't have children or material goods at stake...
1) Show her your next paycheck.
2) Tell her when it has her name printed on it, then she can spend it how she likes.
3) Begin search for new girlfriend. (And get something for that black eye, too.)
Just kidding, of course, but be careful... you may have to "choose" someday.
It's my experience that if things like this are a bone of contention at the "dating" stage,
the incidents only increase in frequency and strength, once well into the "marriage" stage.
You can always try reasoning with her first... she may "come around", and
she may in fact just be picking on you about it. Tell her that having a variety of
planes is far better than having a variety of women, and that they keep you out of trouble
