If 'think about sex' is the equivalent of having 'impure thoughts'...then yes, pretty much every time I'm in the presence of woman regardless of age or appropriateness. As such, these thoughts come up all the time throughout any given day. I'm not one to socialized about it as such thoughts can be objectifying if not offensive.
However, most of my time spent at home alone does not involve such thoughts. Hobbies such as D&D and gaming seldom lead me to think about sex (odd because gaming constantly tries to sell sex in its male and female characters), but rather keep my focus on the activity at hand.
I should also note that I constantly have sarcastic and jerk-like thoughts along with everything that takes place around me as well. This is just the way I think, versus, the way I actually act. Like seeing an elderly lady carrying a food and having the sudden random urge to throw the food onto the windshield of an oncoming car. Of course I'd never actually do something like that...but thoughts or comedic quotes from media constantly go through my head while in public...and far more frequently than the above impure thoughts.