ReineD,
this is going to be one of the few times that I find myself disagreeing with you - sorry.

Quote Originally Posted by ReineD View Post
If you're just going out for the evening to have fun and don't plan on meeting anyone again after the event, then I don't see any obligation to disclose.

But if anyone expresses interest in seeing you again, then of course you should tell, whether the other person is a male or a female.

I have to disagree because the OP posed the question as:

Situation: A pretty and passable twenty something male attends
a vanilla dating event dressed as a girl. She blends in and is chatting with several of the men present, although not leading them on.

Questions: Does she have an ethical obligation to tell a man she is not really a girl, and if so, at what point and how to phrase it?
To me, this means that the person is not getting "clocked" as male and that the men she is chatting with are unaware who they are talking to.
In normal social settings this is a non-issue, since what she has in her panties is nobody's business anyway.

But in a "vanilla dating event" she is messing with people's heads - and that's just wrong. These guys are at least a bit interested in meeting Miss right (or Miss Right-now) and a male posing as a woman is , at the very least, wasting these men's time and it could get so much worse. Suppose some poor unsuspecting dude finds her really interesting or even desirable, what then? How does she tell the guy that she's "not really real" without doing damage - or getting damaged?

In my eyes it's as wrong as a married man attending that same dating event. Both might "pass" but both are misrepresenting to people who have a real interest in the outcome.