is it bad to masturbate over other people when in a relationship?

I have a bf who I love very much but I can't usually get off thinking about him or any males for that matter. Whenever I masturbate I think of girls or guys that I make up in my head 😂 is this normal? I mentioned it to my bf today (not saying I did it) and he said it was gross and cheating. What do you guys think?