The answer is that there is no law prohibiting sex in a car, per se (for example if two people wanted to get into their car – which was parked in an enclosed garage where no one could see in – and have sex in the car – that would be perfectly legal), but if the car is parked (located) at some public place and people can observe what is taking place inside the car, then it may very well be a crime – such as indecent exposure.