I don't think you're being selfish at all. I just think some people just don't have any manners. I'm used of a house full of kids, well they're all "adults" now.

My eldest son has a friend that I cringe when I feed him, particularly when I don't know he's coming. We freqently buy tenderloin and cut it up into steaks. Instead of eating this with sides like a meal, he'll take three or four pieces at a time and make a huge sandwich and come back for more. His lack of portion control is so out of hand that when he shows up, we fix something else for dinner.

I know this sounds very rude, but when you have a $40 cut of meat that should comfortably feed 12 big eaters and it will now only feed 8, there's a problem. We've always had the policy if it's here, you're welcome to it. As you said not everyone has common sense. There were several times when we'd prepare dinner and one of the kids wasn't home yet so I'd make him a plate, announce that this was for X, and it would be gone 20 minutes later when X came home.