Consensual sex in private is considered no one else's business in most Western countries now. That's a good, sensible law, and it holds in most cases.
But occasionally, the definition of "private" is open to dispute. And that can have serious ramifications.
Take the case of a swinger's party held in a hotel bar in the USA. It was infiltrated by undercover cops, and the organizers have been charged because it was legally defined as a public place.