More threads on MR need to be like this.
OP, I'm in a similar situation-- when I'm back from school for the summer I live with my parents and work-- there's nothing wrong with this, despite what everyone says. I have a place at school during the school year, and during the summer I crash at home (~500 mi away), thats what ~130k in student loans does for you.
During high school I was like you, I shagged in the back of cars, on cars, on the sides of roads, on minigolf fields, hell, even in front of a herd of cattle. I pretty much did it everywhere except my parents house or her house, because c'mon, we were in high school. (As a side note; vans = epic win back then, as removable back seats were sweeeet.) But that was highschool.
Now that I've been in college awhile, my apartment is the adobe for sex with the gf during the school year, and I'm in a similar position right now.
My advice?
Suck it up. Cars the last option, especially if you're trying to be "classy", and so is that basement. That should be the second to last choice. Shoot for her house, or if that's her parent's too, outside. Being outside on a clear night is actually really quite nice, try it and enjoy the beauty of nature/the stars. If she's a special girl, maybe get a hotel room, but that really depends more on the girl and what she'll interpret it as.
But that's all assuming I'd behave like you, which I don't. The above are the best options for you, for your preferences and needs.
Below, I'm going to be a pretentious ******* and judge you based on my standards. You can ignore this, as the upper half was what you were looking for.
With that said, some food for thought:
Is having sex on the first date really that important? (The answer is no).
Should you have sex on the first date? (The answer is no, sex driven relationships never work out).
Is sex really important in a relationship? (People debate this all the time, I'd say its important but not paramount; you can't have sex all the time or forever).
Doesn't spending time with her count for more than the fulfillment of erroneous sexual agendas/desires? (Yes).
Does where you have sex really matter? (The answer is no, it shouldn't, it should be more about the interaction, emotions, and meaning than the location and sexual gratification associated with it).
Is judging a girl based on her choice of limited choices of places to have sex even valid? (No, it shouldn't matter what she does/doesn't pick, see the above.)
Are you sleazy for wanting to have car sex? (No, IMO nothings really "sleazy" provided the emotions are there, but judging her is sleazy, and expecting her to is sleazy, and doing it just for the sex and nothing else is sleazy).
But that's just me; there seems to be varied opinions here which is always healthy, as everyone is different and has their own moral, sexual, opinion-driven preferences