Bf looking at other women

Recently my bf and I went to the casino and there was two women beside us who were rather attractive. When they went to walk away I noticed my bfs eyes glance towards their butts, not for long but it still caused me to become upset. I cannot say with certainty he was checking them out but I do know he looked.

I’m curious for all the women who don’t mind their bfs looking at others, how do you deal with it? I need words of encouragement to realize it’s ok