Why does my bf fantasize about women when I’m with him ?

is

My bf and I were in the car today and I told him something and he looked like he was distracted and was like “that’s sexy” and I was confused because I had been tellling him how cute my baby niece was so I asked him what was sexy and he told me he was fantasizing about two naked models wrestling.. do you think it’s a little disrespectful that he’s thinking of other women while he’s with me? Or am I overreacting