Today, my girlfriend and I were walking somewhere and we passed a group of three guys. The guy in the front was kinda in our way and one of his buddies say something to the effect of "Hey Mark, move to the left a little so these guys can pass."

The guy moved over, but as we passed he said some remark to me. I held frame and treated what he said like a shit test. The exchange went back and forth a few rounds and nothing too serious was said. However, once ltr and I were a ways ahead, I barely heard him say some shit about my LTR's ass which could've gone two ways because I couldn't fully hear what he said.

One: he was saying she has a nice ass. If this was the case and our exchange was still happening, I would've said something like yeah I know, try not to get too jealous, etc.

Two: he was making fun of my LTR, calling her fat. She's definitely not fat, but she is thick in a good way.

I left it alone because we were already a ways away, I doubt she heard it, and I came out on top of our little exchange; I wasn't looking to start a fight. That said, I'm wondering about other possible situations.

If I had still been talking to that guy and he had blatantly insulted my LTR, called her fat or ugly or something, what's the best way to proceed? Obviously getting into fights over stupid shit isn't worth it, but wouldn't that be crossing a line?