I am tempted to wonder whether you are serious in asking such a question, or whether you're just playing with me here. If you're mocking me, I forgive you. If you're serious here are my thoughts:
People are entitled to all kinds of naughty fantasies, and if imagining yourselves getting all wild in semi-public places does it for you, then I don't see the harm.
But to act on such fantasies? I think that would be most inappropriate. Let me count the ways.
First of all, it is against Jewish law. As recorded in the Shulhan Arukh Even HaEzer 25.3, it is forbidden to have sex in public places, and is permitted only in private homes.
Second, in most (perhaps all) jurisdictions in the United States it is also illegal. In certain jurisdictions, you could end up a registered sex offender for the sake of a little thrill. Don't be dumb. Absent a major moral claim or protest action like civil disobedience, Jews should follow the laws of the land. This law deserves to be obeyed.
Third of all, it's extremely rude toward others. If the definition of good manners is to help others feel comfortable and treated with dignity, then it would be most impolite to make others feel uncomfortable if they were to see you or intrude on your liason in a public place. What if children happened to interrupt you? How would you explain yourselves to their parents?
Ask yourself why Jewish and American law agree that such behavior is inappropriate. A major motivation, as noted, is that it would make others feel uncomfortable to see you. Furthermore, and most important in my mind, sex is an act of great intimacy between partners. Ideally, it should cultivate emotional and spiritual bonds between you. Those bonds can only be cheapened and weakened by public exposure. Ultimately, privacy is needed to share real loving.
Answered by: Rabbi Jeremy Kalmanofsky