It’s usually the long line of people waiting to use the bathroom that gives you away, and nine times out of 10, it’s a passenger who asks the flight attendants to intervene. Strictly speaking, it’s not against the law to join the Mile High Club. But it is against the law to disobey crew member commands. If we ask you to stop doing whatever it is you’re doing, by all means, stop! Otherwise, you’re going to have a very awkward conversation when you meet your cell mate.