I believe that this is from the same episode, The Dark Ages .
David spends the night in Darlene's room, and Darelene tells Roseanne that she would have heard them if they were having sex. Roseanne tells Darlene that they could have been really quiet so that nobody would hear them.
Darlene replies (something like) "Oh really? You can't."
The audience reaction is priceless.