'what's over the line in one circumstance could be interpreted as harmless flirting in another.'
Yes. You're allowed to greet your friends by hugging them if that's the kind of relationship you have with them. You are not allowed to greet a bank teller that way, or a canvasser, or most of the other people you encounter. The fact that what's appropriate in one situation is inappropriate in another situation is tangential, because...
"the guy must guess where that line is and that he'll be wrong sometimes."
is not really true. First of all, understanding how another human feels in a situation is not 'guessing.' As social animals, humans are excellent at reading cues from body language, facial expressions, etc. to understand how other humans feel.
Second, as a previous reply said, paying attention in your interactions with other humans is actually your responsibility. If you are the least bit perceptive, you are extremely unlikely to be wrong about 'where the line is.' And to go a little further in the you-really-shouldn't-feel-threatened-by-this vein, if you're one of the people who worries about crossing the line unintentionally, you've probably never been near it.
Fortunately this skill can be learned by most of us for whom this is unintuitive. I have had to actively learn about body language cues for all sorts of situations. I can now do it reasonably well, but I have to pay attention a bit and do it consciously. Some people can't learn this, but they are pretty rare, and it is pretty obvious that they have problems with it, and as such, a wider leeway is given by most people out of compassion.
Yes. You're allowed to greet your friends by hugging them if that's the kind of relationship you have with them. You are not allowed to greet a bank teller that way, or a canvasser, or most of the other people you encounter. The fact that what's appropriate in one situation is inappropriate in another situation is tangential, because...
"the guy must guess where that line is and that he'll be wrong sometimes."
is not really true. First of all, understanding how another human feels in a situation is not 'guessing.' As social animals, humans are excellent at reading cues from body language, facial expressions, etc. to understand how other humans feel.
Second, as a previous reply said, paying attention in your interactions with other humans is actually your responsibility. If you are the least bit perceptive, you are extremely unlikely to be wrong about 'where the line is.' And to go a little further in the you-really-shouldn't-feel-threatened-by-this vein, if you're one of the people who worries about crossing the line unintentionally, you've probably never been near it.