Maybe its different when you're driving, vs chilling in the back of an auto-car for a half-hour with your sweety. I think young folks will be very creative filling the time.
I think you're obsessed with this one exception. So I'll state your concern outright: How do you prevent people from having sex in an auto-car?
Well, how do you prevent people from having sex in rental cars? First, people that can get a rental car usually can get more comfortable accommodations for other activities. Second, the rental car is only in the renter's control for a short duration, and that duration isn't cheap. Third, the rental agency will hold the renter responsible for any damages caused by this activity.
I'm sure that Zipcar, Car2Go, Enterprise, Avis, and others have solutions for these situations.
Its not that one thing - its a stand-in for the issue of idle folks in a private space, that they don't own or have responsibility for.
There's no other transportation option in existence that combines privacy and anonymity without the requirement to drive. Nookie is only the beginning of what idle folks will get up to.
Rental cars are not the same - we're supposed to be using auto-cars as an everyday thing, to get to work or school or shopping or anything really. Not just on a business trip.
Actually, I can think of one similar place where people have time on their hands in a private space - public restroom stalls. And look how wonderfully that works out for everybody.