Was I being disloyal?

Long story short: my boyfriend and i work together (we were dating before i got transferred to his location/sector and its only while my department is shut down) and we had a pizza party to celebrate meeting yield. The woman who supplied the pizza is a colleague my boyfriend doesn't get along with. He's mad that i ate a slice. Like he considers it to be unloyal because i "broke bread with the enemy". It was pizza for our entire sector not a lunch date. In my opinion we're way too old for pettiness like this. Am i being insensitive? Was I wrong? I've been trying to see it from his POV but i can't.
Was I being disloyal?
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