If "my" house is actually a meeting place for half the country, or even just half the city, and I've decided almost anyone can paint on the walls, I should not be able to say that one specific person cannot paint on the walls because, for instance, I don't like what party they vote for. Any restriction on wallpainting in that space must be independent of the content of the message. If I wanted to limit that message, I should have to go through the proper democratic circles, ie. pass a city ordnance.
This is the same logic that prevents me from saying that in this meeting hall, gay people are not allowed entry.
This is the same logic that prevents me from saying that in this meeting hall, gay people are not allowed entry.