Is it ok to take her car away because shes withholding sex?

I've been seeing this young lass and things were going ok for a while, so much so that I bought her a car so that she could get to college and back. The trouble started when she wanted a better car, she said that the car I bought her wasn't fashionable, when I said no she said ok no more sex until she gets a new car, WTF? So I took her keys away and said no problem, either get your tail in the air or you get no car at all! She was pretty pissed about that :D Is what I did morally defensible?