These places are kind of a relic of the past. Long ago, before betamax, before VHS, before DVD and before the rise of Internet-porn, there were X-rated theatres (XXX is actually a copyrighted term, and the real rating is X-rated.)
For the most part, these places have gone out of business. Some where closed, because public sex was taking place, and there are laws against it. If caught, even today, you face criminal charges, and may have to file as a sex-offender, which can really damage your life.
However, these places also went out of business, because there isn't much need for them anymore. The ones that do still exist, don't even have real cinematic projectors. They just use standard digital projectors, like the kind you'd see in an office, and its hooked-up to a DVD or VHS player. Its rather low-tech.
Furthermore, those places are gross. I don't just mean because of the presence of public sex. No, I mean, because, if you thought that regular movie theatres were dirty, you haven't lived, till you've seen just how dirty a theatre can get. They don't really clean those places. Imagine 20, 30 or 40+ years of butt-sweat, semen stains, cigarette smoke, and other assorted icky things. If the floors and seats are sticky because of spilled soda-pop and popcorn, that's the least of your problems.
Also, the kind of people these places attract, aren't exactly the most hygienic. How many of these people, do you think has an STD or STI? How many of these people do you think has crabs or lice? Do you think those crabs or lice are regularly killed, for your hygienic protection? If you frequent a place like that, there is a good chance, that you're likely to go home with more than you bargained for.
If you're really this much of a voyeur, there are safer ways to do it. Many voyeurs are turning to technology, such as webcams, to safely and anonymously display themselves. Some frequent more private organizations, that are safer and cleaner than a porno theatre, for example a private party, being held in someone's home, and takes place on clean, or at least you hope they're clean, sheets, but even that's incredibly dangerous. Certainly though, both of those two alternative techniques are safer, and far more legal.
The fact is, police love to raid X-rated theatres. Why? Because if they visit a few times a day, they're almost guaranteed to make arrests, a few times a day, every day, almost without exception. Its very easy police work and these typically aren't violent criminals, which makes it safe police work. I seriously doubt you want to risk being arrested. You probably also want the experience to last awhile, so you can enjoy being seen by as many people as possible. That's part of the thrill for a voyeur. However, the longer you have sex in public, the greater the risk of being caught and arrested. You may get away with a few minutes of public sex, but you're unlikely to get away with a hour or more. Furthermore, those places don't exactly condone public sex. They may be aware of it, but they don't really condone it, otherwise they're arrested as accessories to a crime. Its likely, the employees will ask you to stop, because they don't want to break the law and be arrested.
Now, all that being said, most major cities still have one or two. Chances are, they're in really bad neighborhoods, and legally speaking, they'll be at least a mile or two from any school, church or neighborhood, since they aren't allowed to operate close to any of those places.
Some adult toy stores, adult book-stores and even a few adult video stores, will have a small theatre. I knew a few people in college, who visited them, because they thought it would be sexy, and they ended-up being kinda grossed-out. If I remember right, I think they called them arcades, but they were essentially theatres. Your city may also have an actual movie theatre too, but its likely to be very old, very run-down, very dirty, and in a bad, or even dangerous neighborhood.
Just be careful, and consider using safer, more technologically advanced methods, before visiting an X-rated movie theatre. A webcam is a whole lot safer, and far more legal.
Best of luck. :-)