The key issue is that if someone tells you that they consider your behavior offensive or unacceptable then you are expected to back off. To persist is one way of harassing people. Nobody's asking you to be a mind reader, but it does help if you have some empathy towards your fellow humans.
Let me try to help tomjen3 out. What holdenweb is saying is that it is your responsibility to be psychic. You are required to know, a priori, the current emotional state, the degree of emotional fragility, and the receptiveness to new ideas that your chosen conversational partner possesses.
Most of the "context" of these types of occurrences are judged by the geekfeminist zeitgeist to be centered around the fear experienced by, and emotional state of, the other party -- which you are expected to know through various psychic processes.
For instance, if I am at a party, and people are drinking beer, and there is music playing, common sense tells me that it's okay to ask somebody I don't know to dance with me. If I meet a girl who's a friend of my friend in a casual social circumstance, and we get along, common sense says that maybe I could ask her if she wants to get a drink sometime. If I'm at an atheist conference and it's four in the morning, and a woman gets on an elevator, common sense says, "There is no reason this woman is looking to have a conversation, let alone go out for drinks. Probably she is looking to go to sleep, as that is why many people board elevators in hotels at four in the morning. Likely she is tired, and not super in the mood for conversation."
Perhaps in that situation, if I'm feeling soooo social that I just can't pass up on a chance to make a human connection, I say a small talk thing, like, "I'm really liking this conference!" But just blatantly asking a girl out to drinks? That's aggressive, and unexpected, and quite possibly comes across as annoying or creepy.
Common sense tells us all of this. It is logical. We're all nerds here, right? We love logic? This is basic logic that is easy that you can follow. You can defy these conventions, ask a woman out to drinks anyway, and not be a horrible human being, but it's stupid to say that Watson's reaction to the dude on the elevator wasn't completely justified.
If you don't know how to have a conversation with a person and gague their comfort level, there are in fact courses on this. Take one. Some basics tho: start with a simple, neutral topic. "like the conference?" "great talk! I liked the..." "hi, I'm ... , what is your interest in conference?". If conversation from that point forward is comfortable and easy, feel free to gently escalate. If body language suggests wariness or discomfort or even disinterest, just move on, or suggest talking about it later. You are 50% of that interaction. An interaction needs a 51% or greater consensus to continue. If you take it easy, there would never be problems.
I've been in conversations with women who were obviously uncomfortable with it. Once I spotted it, I politely disengaged and moved on. I have never been accused of harassment. I may have been declared creepy, but whatever the general consensus of my peers is that I am not, so I assume I am not (generally). If some of those discussions I mentioned may have left the woman feeling that way so what? I am not entitled to have anyone think anything else of me but what they do think. Could be I was actually being inappropriate and not realizing it, could just be that the other person was having a bad day, it doesn't matter.
Point being, it isn't that hard to have completely acceptable conversations without any worry about consequence, even if the other person is uncomfortable, so long as you respect that discomfort. Hell, it's even pretty easy to get laid at conferences. In fact, at least in academia, conference sex is a common thing that is openly discussed and joked about in mixed gender company (without resulting in harassment or whatever charges).
Get off your martyr's cross asshole.
As someone else has pointed out, its primary purpose is to reassure those who (reasonably or unreasonably) fear harassment that the issue will be taken seriously if it occurs.
Understanding the mental state of the people around you is called "society". It's not particularly easy, which is why it takes us years to learn it.
But if you need a simple heuristic to not get in trouble at a conference, here it is: do not make sexual advances to people unless you are 100% sure they are welcome. Or, if that's too hard, use this one: do not make sexual advances at conferences. Seriously, it's not hard. You can hop on OK Cupid when you get home.