Gossip, as we all know, provides the occasional tasty titbit of useful information, and, properly done, causes minimal harm whilst providing no small entertainment to those involved. But in the pursuit of gossip, as in all other such things, the guiding principles of good manners must remain firmly in mind. Instead, it serves to guide us in choosing the appropriate time and place in which properly to indulge our own particular tastes. Hence, it is perfectly appropriate to eat with one's fingers at a Fourth of July Picnic. It is likewise quite proper for adults, should they so desire, to indulge in any number of a large variety of physical activities and personal intimacies behind closed doors.
Problems arise, however, when the latter activities occur at the former event.
This, of course, has the immediately obvious, and decidedly negative, effect of interfering with the pleasures which one might normally obtain. But, of far greater concern, it also brings with it the potentially discomforting possibility of making other people voyeurs against their will. Assuming I properly understand the point raised at the end of your second paragraph, I believe it important to maintain that most lapses in normally good manners, arise from unintentional error. When confronted with such happenings, I most strongly recommend that you assume, in the name of good manners itself, that a regrettable, and indeed immediately regretted, totally unintentional error of some sort has in fact occurred. Please, I beg of you, do not feel obligated to point out said, now sadly public, error in a similarly public way.