Let me say this in no uncertain terms: there is no manner of dress or flirtatious activity that gives you the right to initiate unwanted contact with another member of the convention! This is behavior that is unacceptable, period. Full stop. End of sentence. No mitigating factors needed or even allowed. I don't care if you have watched a young lady kiss every single person in the lobby on her way to you, when she gets to you, you do NOT have implied permission to initiate contact. You don't get permission to touch, hover over, leer at, or otherwise harass her. I don't care if a guy has been talking suggestively with you for the last hour, you don't get to grab him without explicit permission.
And that, really, is what it comes down to… explicit permission. There is no such thing as implicit permission at the local conventions. No clothing choice or activity implies that you have any permissions that have not been explicitly stated. Silence in no way implies consent, silence is dissent. Silence means NO!
These events are billed as a safe place for a normally ostracized group to be able to “be themselves” and “feel safe” in their geeky, crazy, often socially unacceptable interests… and it is high time to make that true for all of its members.
I want to emphasize that whole list of what you should not do without explicit permission: touch, hover over, leer at, or otherwise harass someone. I believe we should have a community culture where, if you see someone doing this to someone else, if that person complains or asks for help, you should take them seriously, and respond with support. Even if someone is not complaining but looks distressed, it is all right to check with that person to make sure they are ok. And someone who persists in harassing behavior or is snide or disparaging about having it pointed out (as opposed to apologetic) should face anything from a stern talking to to removal of their membership by convention ops. But also, as Jer points out, possibly a more significant thing they should face is vocal intolerance of such behavior by the community. In other words, we should all work on speaking up when someone does or says something inappropriate.
A friend of mine recently recounted to me a situation that happened to her within the past year that involved a sudden, uninvited come-on from someone leaning into her in such a space that she was trapped. There was no doubt she felt assaulted. At "How to flirt" panels at cons we specifically counsel against "trapping" someone you're interested in as well as getting close into someone's personal space without invitation, but thinking back on it I think we've tended to emphasize the "this is not a way to make friends or flirt with people" aspect of it, not the fact that once you are physically trapping or intimidating someone, if you touch them in a way that they have not given you permission to, you are over the line from "stalking" to "assault" and you might cross the line into harassment earlier than that depending on what you are saying to them.
It will be interesting to see where this project goes in terms of defining behavioral boundaries. It's interesting for me to think about, for myself. Often comfort with one sort of behavior depends on the situation. For instance, I have a number of people I enjoy giving a quick kiss hello or goodbye - it's something my family does, among ourselves and with some extended family, and some of fandom is my extended family. Yet, if someone gives me a hug, and holds me in that hug, without relaxing so I could easily step back out of it if I wanted to, and when I look up at them, goes to kiss me, it's not ok. Because I'm trapped. Sure, I could get out of that hold. a) it's not particularly tight, just firm, and b) I know martial arts. But it's taken me some contemplating to figure out exactly what bothers me about this sort of situation. Kissing the friend doesn't bother me. Feeling trapped (physically and socially) does. The psyche is an interesting thing. (When I was in junior high I wouldn't let any but the closest trusted friends put an arm around my shoulders, because that was too close to having an arm around my neck. I've come a long way since then, though anyone who knows me well might note that I still pretty much avoid clothes and jewelry that come tight or close around my neck. People might have buttons you don't even know you could push by touching them in certain ways. So check.)
I believe full contemplation of eliminating harassment at cons should include harassment on the basis of race, religion, sexual or gender identity, size, age, and disability. But that doesn't mean effort on any one of those fronts is not worth doing in and of itself. Please participate.