He wants what he can't have.

"The answer is simple, many studies have shown that people will want something more if it is restricted to them. Your feelings for her were in part because by being in a relationship, she was out of your reach, and now that she is together with you, you two are likely doing it whenever you fancy."

"So in order to rekindle what we have, we need to, uh, break up?" he asked, not willing to go that far just because he had some weird kink.

"Oh no, you don't have to do anything so drastic to get back to a healthy relationship. Which isn't to say that you won't find what you have to do hard, of course, that's the point of it."