In The Dreamers when the parents of Isa and Theo saw them, they wrote a cheque and gone. What was that? They didn't want to face up to children or emancipate them?
I don't think the parents were startled by seeing Theo and Isa sleeping naked together because these two characters, Siamese twins, portrayed very strangely in the movie, like, they sleep naked next to each other, they bath together and so on. But what definitely shocked them is seeing Matthew in the same condition alongside them.
Even though the parents have witnessed a scene as unpleasant as it can be, they're still their kids, and you know how parents are so liberal towards their children now-a-days, "They're kids and it's life" kinda attitude.
So, the parents might be thinking not to embarrass one another by waking them up and the parents left the cheque because their kids are still college students and they don't earn anything for a living.