Or an attractive coworker or friend?

Bottom line, what are you going to do, tell people there are "bad thoughts", and that it's wrong to fantasize about somebody? I mean, there are certain Very Fucked Up religions that capitalize greatly on the notion that certain thoughts (as opposed to actions) are in and of themselves "bad".

(Ofc. it's almost certainly creepy to tell someone you don't have a sexual relationship with that you think about them when you masturbate.)