So she lets you spend your own hard-earned money on what you want (all reasonable purchases btw), she lets you play rugby, lets you go to the gym, lets you go fishing, and that makes it okay for her to drive around the new Denali you bought for yourself while you're driving around her little Focus? And the justification is that she gives you sex in return.
You should be able to spend your money on whatever you want, as long as it's not irresponsible and not making you guys broke. The money you spend on stuff and the time you spend doing those activities are all very reasonable, smart ways to spend YOUR money and time.
And you should be getting all the sex you want without buying her a car. It's a physical expression of love, not a (expletive) reward. She's got you trained.
Nothing wrong with spoiling your girl, or buying her a car. But when you buy yourself a car and she thinks it's ok to make it hers, and you let her, something's wrong with that picture.