So recently DH and I had a few family members over for dinner. While it was finishing cooking, we put out some cheese, crackers, napkins, and plates in the living room. Our guests proceeded to eat the cheese and crackers while sitting on our couch... without using plates. I was sitting there watching crumbs fall on the floor, under the couch, in the cushions, and I didn't know what to say. It feels presumptuous to tell someone twice my age to use a plate to catch crumbs. At one point I said something along the lines of "don't worry about making more dishes or anything - that's what the dishwasher is for! Feel free to use a plate if you like." The response was something like, "Oh, I'm okay."
Is this expected behavior? Is it okay to get crumbs all over when you're at someone else's house? Or can I say something? And if so, what can I say that doesn't come across as condescending or rude, but makes it clear that I expect a plate to be used?