I think it's normal, what else could you be thinking about while getting your ass played with!? Wondering what it would feel like to have a real one inside of you, all the while getting them off with your ass. Very hot if you ask me.
I would love if my wife gave it to me everyday, but I don't want her to think I don't enjoy her. I do enjoy her and we have great sex, but I love having my ass played with, with a nice sized vibrator