I figured most women would say 'no' to this while most men said 'of course!' It seems I'm not far off. I think the imagining people naked thing is definitely more common for guys but yes I do it to.
I don't walk around imagining everyone naked (I don't think I'd want to!) but if someone catches my eye or I'm watching someone for a decent amount of time I usually will start to wonder. Like today while I was using one of the ellipticals across from the racket ball courts at the gym.