I beg to differ. There are private groups in existence right now, who are excluding protected classes and everyone is perfectly fine with it ( sex ). I don't even have to troll 4chan. They are right there in the open[1]
I will admit to not understanding the scope of the "discrimination" against protected classes that is barred by law. It clearly has some significant extent, but I've never really been able to find a clear description of where the edges are.