Yeah, I thought that was such a great touch. Because it's not even just about how viewers might react in the same situation, it's that it's such a cliche for the parents to be all OMG THE CHILD WALKED IN and it becomes a comedy for several scenes. But in this case they're just far too used to being somewhat performative about sex to freak out, so it was all on Paige. The fact that it might keep her from checking up on them in future was I think almost an afterthought, because the bigger issue for them was realizing she was checking up at all. Before that they really didn't seem to even think that was a danger since their rules had always worked. Certainly not enough of a danger to spend however long every night lying naked and staring at each other's genitals in case Paige walked in. (Not that they were just staring when she walked in.)