IMO, Julia just started to tip her toes in the waters of the swinging lifestyle.
How about Julia marries Peter and the couple agrees it was just a crazy faze they were going through before they tied the knot. But after awhile... Julia starts to miss all the attention (and the wild sex), while Peter's new job becomes so demanding that he's rarely home, and even then, he's spent. So he sets up a secret video surveillance in their home to "keep an eye on things" just in case, while he's out of town. And he had admit to himself that he is missing the heady feeling that he had secretly watched Julia.
maybe, like this