His and Anna's disinterest comes directly because they don't want to be outshined again and put in a position where they have to say no. It's very poasible that Alex can't do it because he doesn't want to to see Anna have an orgasm with another man, but I think they do swap outside of the apt. So, I think it's because they know that they make more money in the long run if they don't do it. We always tune in even of we get disappointed. Once they start swapping it turns into a snowball and viewers will always expect it and the appeal will be lost. The hook is hoping something will happen, not knowing something will happen.