Fiance's friend showing nudes to him

Me and my fiance have been together for 2 years, he's my person. I love him with all my heart but this made me feel crushed.

He's currently taking engineering and has a few friends in his class.

Recently, a friend of ours was over and my fiance starts telling him about these nudes his friend showed him and how ugly her boobs were.

I immediately felt hurt but tried to stay calm. I eventually told him how it made me feel and he brushed it off and acted like it wasn't a big deal. Which I agree, its not huge but I just didnt understand why he wouldnt tell me and why he even looked in the first place.

He's very big on not cheating, not going to clubs when he's in a relationship, no talking to other girls. I think that's what shocked me. He was never the type to do something like this.

Am I in the wrong here? Am I just being insecure?