Is it normal when you think of someone else when you're doing it with your partner?

Help! is it totally normal? I am in a 5 year committed relationship. We were doing it the other day and an image of someone else suddenly pop into my mind. I dont really have a huge crush on him. But I am attracted to him though. I don't want to cheat on my partner just by thinking of someone else while we're doing it.. I love him and we've been together since we were in college.