Is it normal that i frequently talk to ppl and visualize genitalia?

I used to have this habit that started when I was fairly young where I would look at a boy and think about what his penis looked like...was it big and hard or soft and small. I would tend to assign the big ones to the boys I found more attractive or thought to be more aggressive. As I got older, I do the same but now I also am picturing female parts...and I do this while I am talking to people (my job is with the public so it can make my day interesting). I really dont have much of a problem with it except when its people that might not be appropriate to picture naked but Im not sure if its normal. i dont seem to have a lot of control over it and the more I try and not do it, the more I tend to.