Serious EnglishコミュのYale Prom joke question

Hmm....I know they say it's boring to explain a joke.

Sometimes I get these Quote of the day things...but I don't understand it sometimes.

Well, never really bothered me until today. Not that it particularly bothers me today anyways, but you know, the thirst for knowledge overcame my laziness.

So here it is:

If all the girls who attended the Yale prom were laid end to end, I wouldn't be a bit surprised.
- Dorothy Parker

I don't get the joke. Why is that?
...

So the literal meaning means, to the person who said this: It is quite normal if all the girls at the prom end up sleeping with someone. (check)

So?