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.