Was I in the wrong?

So my girlfriend has been staying at a guys house a few nights a week. The guy used to be my friend and he currently has his own girlfriend. One night she asked me if I could buy her alcohol, I of course said no because she was 20 and she was staying at his house that night. She then replied with "Ok, I got other people, I was just asking you first". Keep in mind the guys girlfriend is over 21 so she could buy it, but my girlfriend said that she doesn't buy it for him because she doesn't want his mom mad at her. After her comment, I got really pissed and broke things off because she was already spending more time with him than me (4 nights a week).. What do you all think?
Was I in the wrong?
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