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Sure. But just because something is self-deprecating and a (just barely funny) joke doesn't make it acceptable. Context matters, and this was literally the first thing the guy said to me.

It's not at all appropriate in a business setting.

I guess it depends whether you see tech events as business setting or social setting.

Also, wouldn't the aforementioned dress code be deemed inappropriate in a business setting?

If you're there for work it's business. (and what's so inappropriate about a leather coat and black shoes?)

"Normally when I'm hanging out with someone that looks like you, I'm playing Dungeons and Dragons." Is that purely self-deprecating (assuming playing Dungeons and Dragons is such..)?

It is extraordinarily creepy.

No, it's an implication that the OP is dressed like a prostitute. That's not cool.