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[Bluenorthwest]

124. I've never seen a figure drawing class that allowed physical contact or leering at the models.

Did yours allow that sort of thing?
A list of 'rules' from a site about figure drawing:

1 Don't touch the model
2 Don't talk about the model's appearance
3 Teacher directs the session NOT the model
4 No talking
5 No flirting or acknowledgement of the attractiveness of the model.

Now, the party in question was like such a class in what way? They touched, the entire point was to enjoy the models appearance, dancer directs and conducts the session, the viewers talk freely to one another and to the performer, and not only do they flirt, they leer, make suggestive comments, grope, get groped...
Another difference would be that the dancers were 'of a type' that is a figure drawing class would as you say, offer up an 'old dude' not two women of similar age and type. Of course those boys and that mom would not get off on an old dude, and the entire point of the party was sexual excitement.

http://www.thenakedshed.blogspot.com/2006/05/rules-of-life-drawing-codes-of-conduct.html