Boyfriend doesn't like it when I call him bro?

My boyfriend and I are in a new relationship and I say bro a lot. I can't help it because I picked it up from some where and i've been saying it. He says its disrespectful that I keep calling him bro but I don't understand how. I called him bro when we first met and have been calling him that ever since and it hasn't become a problem until now that we're actually together. He is just mean about it. I tried to explain and tell him that i call everybody bro and he flips it on me and say "so I'm everybody?" and it just makes it harder for me to explain myself. I have a bad habit of saying it. When I say it he gets irritated. he told me with a hostile tone in his voice "I'm your man , not your bro. I'm your boyfriend, not your bro." but obviously when I say bro I don't mean that he is literally my brother or anything. I'm just saying it like dude or whatever. I don't see how this is disrespectful and I won't stop because I don't see how its a bad thing. whats the big deal? The word is just in my vocabulary and I call everyone that. how is this disrespectful guys?
Boyfriend doesn't like it when I call him bro?
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