Good question. What exactly would be the problem with doing what you said? I suppose, the problem would be that people have spent effort coming to your party, and had some expectations about it, but instead something unpleasant was shown to them. You violated their reasonable expectations (most parties are not flashing parties) and spent their effort in vain, so they have every right to be angry.

This isn't usually the case on facebook: there's almost zero effort in reading somebody's post, and you usually have pretty good expectations of what kind of stuff the person posts about, so you either keep them as a friend if you like it, or you don't if you don't.

Do you see the situation differently?