Is it appropriate for a supervisor to flirt with their employee?

I am curious. I used to work at a retail store, and one of the managers is dating his employee. It kind of made me a little curious to the legalities of that situation. She was on my level, so I thought that was strange. But I am curious, is it appropriate for a supervisor to flirt or even have and show his attraction towards his female employee's? At the end of the day we are human beings, ual beings and we cannot necessarily help this. But is there an actual law stating that employees cannot have any relationship further than work related? I am not sure, I live in Canada so.. I am curious..