ETA: The guy I describe in this blog post wrote to apologize.  We’ve been having a kind and productive conversation in email, and there’s a whole ‘nother post’s worth of material there, about the importance of open-hearted communication.
So this happened: We had an open house for our feminist hackerspace, and this dude showed up (along with a woman who’d been to AdaCamp) and told a story about a guy jerking off and eating his jizz and wondering whether that made him gay. This was after he had monopolized the conversation continuously, talking three times louder than anyone else in the room. After he’d walked around the room, snarking on all our little signs, reading them out loud in a mocking voice: “Fake broom closet!” “Library!” “Quiet area!” “Carefully framed anti-harassment policy! <chuckle>” To which I replied, “Yes… that’s where we’re going to hang it.” 
“Nah man, that’s cool, I like all the signs, it’s got a good pretend-play vibe.” No one commented, so I’m guessing that the enormous clouds of steam coming out of my ears at this point weren’t visible.
In the lead up to the masturbation story’s climax I was actually disassociating — while he was talking I wandered around wondering to myself, “Is this really happening?” Then I heard voices in the hall — some of our “junior members”, age 10 and under. That’s what made me do something. I thought, “Oh, NO. They do NOT need to hear this bullshit!”
So I walked up to the guy, interrupted him, looked him in the eye, and said, “I did NOT work hard to start a feminist hackerspace to have some man come in here the first night and talk about jerking off.”
“Hey, I was just telling a story to my friend who also works for SFSI [San Francisco Sex Information] — I’m sorry you were offended”
“Yes, and you were telling it loudly at the open house for a feminist hackerspace!”
“It was just something that happened at SFSI….”
“That’s great — when you have an open house at SFSI, feel free to tell as many stories as you want about people putting things up their butts, too. Please don’t do it here.”
“Whoaaaaa.” [Hands put up in the air in the universal "I'm unarmed" gesture]
In his “apologies” he kept talking about how *I was offended* — not about his having done something offensive. Everything he said made clear that I was a prude who was spoiling a fun conversation. He wanted to know “can we get past this?” I said yes, but I wish I’d asked him to leave. I was incredibly uncomfortable until he did leave, and I walked around and marveled to myself about the chutzpah it takes to stay somewhere when you’ve made an ass of yourself — it seemed like an eternity before he left. I kept walking around the space because I was too agitated to sit — I didn’t feel safe or relaxed. But it hurts my joints to walk around — I ration my walking pretty abstemiously. I’m incredibly pissed off that I wasted a bunch of my walking-around energy on being agitated in my own hackerspace!
I’d like to control my temper if/when this happens again; it might be more effective to just say, “One reason we have a feminist hackerspace is because we don’t want to hear _______________.” If I can say that calmly, it seems like a more straightforward objection than my highly emotional one. But I’d also like to give myself a break; I was angry and triggered, and very surprised that the interaction was occurring at all.
While he was there, I kept wanting to go over and give my bona fides as a sex-positive feminist, as a speaker at Arse Elektronika, as a writer of erotica; I felt like I’d just provided the punchline to a thousand rude comments about humorless feminists, that *I* was the one who had something to prove. What fascinated me most about the whole thing was that I was only moved to act when I heard the kids in the hall. It seemed obvious to me that they shouldn’t be exposed to this boorish man and his lack of judgment. Up until then, though, I was tongue-tied; I had not yet arrived at the thought, “Hey, *I* don’t have to listen to this here!”
Everyone else was awesome and backed me up; my anxieties are my own. I’m only 42. I hope to have a few more decades to work on this stuff.
ETA, 10/4/13, 7pm :  Said dude has written to apologize, and did so very graciously. For me, writing this post was about processing what happened and how I felt about it, not about wanting to vilify the guy who did this. As I said in my reply to his email, I might well have found his anecdote hilarious in another context. His apology in full:
I am that person from last night. Let me start by saying I’m sorry. I think what I said was “I’m sorry for making you feel uncomfortable”, but if I said I’m sorry that you’re offended, that makes me a quadruple asshole and I’m quadruple sorry. I have a crazy (horrible and stressful) personal situation going on right now, so I can’t give you the respect in writing that you currently deserve, but I would be more than willing to have a conversation with you if you were open to that. I’m sorry.
 Quotes are my best approximation of what was said, but I can’t claim 100% accuracy.