It is such a shame that society in this country is not mature enough to cope with nudity in the same way that the rest of Europe (and many other parts of the world) do. It is because of this that nudity and sex are inevitably, and wrongly, associated. Thus, people having sex on the beach are assumed to be naturists and vice versa.
Thats exactly the problem, the mindset is changing, albiet slowly, but the ones we have to put right firsat are the miscreants who reinforce the nude=sex minset. If you see any inapproriate behaviour on a beach, do something, don't pretend its not happening.
"What can I do" I hear you ask, well you can;
Shout at them and draw attention to their misdeed, and bring shame upon them.
Take photgraphs of them, or pretend to with a mobile phone, even if it doesn't have a camera.
Go over to them and tell them to stop, if you feel its safe to do so.
Do anything, but don't let them drag naturism into the gutter.
Pete