I Genuinely think this is not really a Geek problem. There is statistically large number of people with sexual deviation. For instance, it is estimated that between 3-9% of the population are Pedophiles. At a conference with about 100 people there maybe 3 persons there with a sexual deviation. But it only takes one of those guys to approach all the women in the conference and spoil the whole thing for all of us.
My suggestion is if ever you were to encounter a situation of sexual assault report it immediately to the police[not the conference organizer]. I don't want that guy who hits ladies on the bum to be walking around freely in society. He would not be operating at Conferences alone.
As a side note to how important it's to report these things:"nonincestuous pedophiles who molest boys had committed an average of 282 offenses against 150 victims". As I said previously, just one deviant can crash the party for a whole lot of people.
This can be due to a lack of respect for women in general, but also due to a mix of social ineptness and sheer desparation. And you can't tell me the latter is not statistically more likely to occur in geeks.
But putting your hand down a females underwear and hitting someone's bum unexpectedly. Normal people don't do that.
People who are going with the flow of a conference often want to stay up and hang out until 10 or 11 and meet people, and it's not difficult to organize or get local companies to organize events that go that late, like hack fests. People who decide to stay up later, well, at that point they're not going with the flow of the conference. Hopefully they know what they're doing. And if they don't, and something bad happens, of course the offender should be blamed.
I just don't think having to wing it should be a normal part of a conference experience.
Edit: I realized that I've only been to small (<500) conferences. It still seems like bigger conferences could have evening events, but they might be trickier to plan. Perhaps having a few evening events that are announced and that the organizers can vouch for would be a solution. Also I don't think it's all-or-nothing; the more well-organized activities there are that fill up the evenings, the less chance of something bad happening at a poorly-organized evening activity.