I sat on another guys knee in picture, and boyfriend doesn't like it?

I was at a gathering recently of people in my university project group, and as part of a group photo, the other five members of my team sat on the sofa, arranged as guy, guy, guy, girl, guy. Since my guy friend was in the middle, I jumped in and sat on his knee, and slightly on the knee of my girl mate sitting beside him and I leaned back so I was more on level with everyone else in the picture. He them put his arm around me and the picture was taken. My boyfriend is now seen the picture and isn't happy about it. He says that I shouldn't be sitting on another guys knee, friend or not, and the guy looks too cozy and enjoying it too much. Is he right? Or is he being unreasonable?