This seems like one of those problems without an answer.
As you've made clear you already know, pretty much any place you have access to, besides your houses, is "public," so you shouldn't be messing around there. But you can't mess around at home, either (I definitely understand that - I'm still not allowed to have my boyfriend in my room at home, even with the door open).
The motel thing seems obvious, but there's that stupid 21-thing, in my area, too. My friends and parents and I have all talked about this ad nauseam, as it's come up a lot in relation to weekend holidays and New Year's parties.
We decided that it's probably not law, as you're an adult - it's probably a policy to make sure you pay for the room, and one hotel told me it's because their room service serves alcohol and they don't want to be responsible for people who can't drink legally.
So what are you going to do? The only thing I can think of (besides going over to a friend's house, which is kind of tacky) is just to 1) talk to your parents about it, however uncomfortable that is going to be, or 2) wait till your parents are gone for a little while. Neither are fantastic, but the first one will get you into a lot less trouble if someone comes home early.
~lemming, Scarleteen Advocate
this is what you get for liking it.
"The best-looking boys are taken, the best-looking girls are staying inside. So, Judy, where does that leave you, walking the streets from morning till night?" --Belle and Sebastian, "Judy and the Dream of Horses"