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What would you do?
Well, I would start by first being aware of my surroundings and who I am hanging out with to the best of my abilities. A girl who has a bunch of stuff left over from an old boyfriend ... such as his house ... is probably going to end up being paid a visit by said guy, and I probably don't want to be there for that.
If said girl is worth it - I would probably try to first resolve the issue by trying to figure out how to give the old boyfriend his stuff back to avoid the conflict in the first place.
If that somehow never develops, and this situation boils over into the case where he is breaking in and this girl is worth it, then he'd probably regret flailing his arms around and exposing his neck. We would negotiate his property affairs under the supervision of police and/or court authorities. He probably never would have gotten to his whole alpha male speech.
If this caught me by complete surprise and I have absolutely no skin in this game at all - then I'm probably just going to shrug and walk off.
I kind of have trouble seeing me in the situation where I'm in my girlfriend's house and I wouldn't feel as though she is worth fighting for. Though, like I said, I would have probably tried to figure out how to avoid this whole thing in the first place. Or, at least, make him figure out another excuse to use for his behavior, there.