One sexy scene that comes to my mind:
A couple is at a party, and they decide to go outside and stop by the window, so they can still talk to people if they want to but at the same time they can do some foreplay hidden from other people's eyes, who can only see them from the waist up.
They get hornier and hornier until they cannot stand it any longer, and the guy penetrates his girlfriend (pussy or ass should be available options). He is hugging her from behind and their in and out movements should be controlled not to call the attention of everyone else.
For me what is really sexy is having both really interested in having sex with each other, and both horny to the point of taking risks like being caught. So it is not really the place that matters.
Other sexy scene would be a guy and his girlfriend in the living room of her house. whenever they get alone, they start to do naughty things, until they come to the point when she is actually lifting her skirt, and lowering her panties to let him to penetrate her and to start to bang her. every time they hear footsteps or any noise at the doorknob they stop and fix their clothes.