My very frank self would have probably said something quite generic, but also quite true, "Well, when it comes down to it, we all know that only the partners truly know what goes on in a marriage" as I left the room or gathered my things for my departure. It might have given everyone pause.....but I doubt it would have stopped them. But then, I'm part of the older generation of which you speak, and am married.
Flickan, please excuse me if I am wrong, but I infer from your post that you are not married, so such a statement from a singleton might not have had much cred in this situation. I doubt there was much you could do other than absent yourself from the conversation, as you did. Your retrospective insight that you might have invited the teens to accompany you elsewhere in order to remove them from the conversation is on point. Jeez, look at the lesson those kids just learned!