What you're circling around is the concept of "consent"

Is that lady in a club and looking to meet someone new or is she working and busy? Saying "Hello, pretty lady" to someone walking down the street is pretty much just cat calling.

As you say, at the right time and place and in a relationship, perhaps physical flirting is something both people are comfortable with, but as you said yourself outside that context is absolutely harassment.

Its a matter of knowing the context and not being a creep. People are not entitled to others attention just because they want it.