"The possibility of being caught or seen is a huge turn-on for me. We don't have to have full penetration, but getting close works great. I've started going out without panties on so he can start things wherever we are."

She might not want to actually have sex in public (getting arrested isn't good), but the thrill of being observed is a huge turn-on for many women. After all, part of the excitement when you're a teenager is not getting caught doing... whatever. Why would that change in adulthood? 

To the guy who thinks his girl "wouldn't be like that": Yes, your girl, too.