One of the most fascinating things about the sex is that there is this species that "consumes" sex in the, from the evolutionary point of view, strangest of ways. Of course, it's the humans. Let's think about how we have sex from the point of view of some other land mammal, let's say a dog. To most of our earthly companions, the act of sex is something you celebrate your victory with: to the male, it's the triumph over all the other "lesser" males, to the female it's the proof that she is the most suitable AKA the chosen one. We, on the other, hand find public sex not only not-proper but a form of deviant behaviour. Imagine what your dog is thinking when you go upstairs to your bedroom in the middle of the night, turn of the lights and cover yourselves when having sex. He's probably thinking what the hell is wrong with those guys they are acting as if they are doing something wrong ;)