he made an assumption with very little info
In no way was his response appropriate but seeing your GF flirting with another man isn't little info. One time I just started seeing someone we agreed to meet at a bar, we talk for a bit and she excuses herself to the washroom. On way to the washroom she stops to chat with a guy she knew, they were obviously close and were really touchy-feely - they go on to open-mouth kiss... It was the second date and I knew she lived close so I just upped and left. Texts me soon after asking where I went? I left because I saw you making out with that guy. Omg he's my gay best friend, screenshot of his insta - a very gay man. She comes over and we bang. Yep thats the story just here to brag B)