My parents are similar to yours, except they don't' believe in sex or cohabitation before marriage. Me, being a child of the modern world, don't agree.
I'm 26, and I live at home while I'm saving to buy a house. When my parents are on vacation or away for the night, I invite my BF to sleep over. Given their morals, when he does, I simply don't tell them.
If you know your parents have a problem with your BF sleeping over, why are you telling them he's sleeping over? A simple omission of that fact can save you a lot of hassle and grief.
Unfortunately, even though you are 18 and out on your own, your parents are holding the fact that they're paying for your schooling over your head. This isn't right, and my parents did this while I was in college, and I absolutely hated it.
What you have to decide is this: is it worth it to you to have full disclosure with your parents about him sleeping over and having to foot the bill for your education? Or are you simply going to say "what they don't know can't hurt him" and leave things as they are? Given the state of the economy, I would say let you parents pay for college, and make the smart decision.
If they ask you whether he's sleeping over, don't lie...but start spending nights at his place, so if you say "no", you're technically not lying. Unfortunately, as long as we live with our parents or they're footing the bill for us, we're at their mercy and their rules. When you get out on your own, you'll be at the mercy of your landlord or your boss who will impose ridiculous restrictions again. Adulthood doesn't mean you get away from that.