Offered to bring my boss a sandwich

And my fiancé flipped the fuck out because my boss happens to be a decent looking guy. Now I can tell you I love my fiancé and have zero interest in my boss like that and the only reason I offered to bring him a sandwich was so I wouldn't get crap about taking a long lunch break. I told my fiancé this and gave him full access to my phone and he still flipped a lid. I have never cheated on him and never would.

So tell me, did I do something wrong here or is my fiancé overreacting?