Uninvited Guest

When is it okay to come over to someone's house without calling first? Is there ever a time?

I think it is just good manners to call and say "hey, I am in the neighborhood, I wanted to see you, can I stop by?" Am I old fashioned?

I was talking to a friend and there was a "situation" with her roommate. Her roommate is "dating" and when I say that, she is not in an exclusive relationship. She goes out on dates with different men trying to find the right one. Well, she had one man stay the night with her. The next morning, she had an uninvited guest "stop by" while she was in the company of another man. The person who stopped by was not invited, didn't call, he just showed up. He was knocking on the door and calling her phone demanding to see her.

To me, that seems a bit stalker-ish. Why would he think it was okay to stop by like that? But it made me think, when is it ever a good idea to stop by without getting approval first? If she was all alone, would it have been okay? I think people need to set boundaries.

My thoughts on the situation:
1 - It is never okay to come over to my place without calling and getting the "okay" first. Why? I might not be home, I might be busy, I might be getting ready to leave, or I might have something else going on where I do not have time to deal with an interruption. Not that I am doing anything wrong, but there are times when people need their space.
2. If someone does decide to stop over without prior approval, set the boundaries. When you answer the door, tell them "I appreciate you thinking of me and coming over, but you need to respect my space and give me the courtesy of a phone call. I am busy right now and can not entertain your company. Talk to you later, have good day, bye."
3. If someone stops by the 2nd time without prior approval, do not open the door. Either call them on their cell phone or talk to them through the door and tell them "I talked to you about this before, you really need to call before stopping over. I can't talk to you right now, bye."
4. If they do it the 3rd time, do not acknowledge them at the door at all and you probably need to separate yourself from this person, they might be a stalker.

So, when is it okay to stop by without prior approval? This can get tricky. I think that even if you are in an exclusive relationship, it is still courtesy to call first. I guess the only time when it would be okay is if you have your own key to their place, but if they are not expecting you, I think it is still nice to call and say "hey, I am on my way over."

The problem occurs when someone stops by without getting prior approval and you welcome them into the house. When you do that, you are sending them a message that it is okay to "pop up." Even if you tell them "you need to call next time, but since you are here, come on in." That would be leaving the door open (no pun intended) for them to just pop up anytime they feel like it.

What is your opinion? When is it okay to just show up?