Why did my boyfriend take his friend's side over mine?

Anonymous
I have been dating my boyfriend for approximately five years. He has been friends with said friend for approximately two years. I have been through a lot with my boyfriend and we have managed to get through it all, from a disapproving family to cheating and breaking up and getting back together and we are in a much better place than we ever have been. We are living together for the second year, and last night I got into a fight with his friend. The three of us went out to a putt-putt tournament/bar crawl, and then we went back to mine and my boyfriend's place. My boyfriend went to the bathroom, and his friend started messing with all my stuff. I asked him to stop over and over and even moved my pet into the bedroom to get him to stop messing with him. However, he followed me in there and I had to physically remove him back into the living room. I told him that he was really pissing me off and that he had to leave. He told me that he didn't think so, so I went and got my boyfriend. My boyfriend basically yelled at me and I told him that if he left with this friend, then that I was going to move out and we were breaking up. I have had it with this friend, and this is not the first incident. He told me whatever and left with him, so I took my essentials and am planning on going back sometime next week to get the rest of my belongings. He hasn't even contacted me yet. Why?
Updates
+1 y
in response to some of the answers, cheating is not always the be all, end all of a relationship. Sometimes people make mistakes, and why the f*** would you answer a question if you are just going to be an a**hole? I asked this question because I am unsure of what to do in this situation. I genuinely was looking for some insight to what his mindset was, but if you are going to act like a f** and be unhelpful, then f*** off.
Why did my boyfriend take his friend's side over mine?
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