It is absolutely unprofessional for a man to hit on a woman at work; if it is bothersome, a woman should take it up with her superior immediately because it is both inappropriate and creating a hostile environment for women to work in.

I am saying this as a victim of continuous harassment at the two jobs I've worked at, as well as a victim of sexual assault.

However, if it's harmless and something that isn't bothersome, it's best to tell the guy to call you AFTER work, or to keep work at work, and pleasure outside work. I've dated a coworker/shift manager before, but we kept it low-profile on the clock.