This entire thing is about rules and pushing boundaries and exploring both their own and their partner’s edges. Talk of the “rules” and whether either of their behaviour “breaks” it but for the most part there is never a breach. Constant discussion about consent. And things keep escalating. Of course you don’t get the almost constant internal monologues about questioning their desires and behaviours and what those actually are that are in the light novel. These two goobers would be WAY down the pipeline before either of them even consider that their behaviour even MIGHT be considered mutual desire.
The “official” rules were actually few: The biggest was Miyagi pays 5000 yen for a set amount of time where she can order Sendai to do whatever she wants within certain perameters. What happens in foot licking club stays in foot licking club, so no telling anyone about it, no contact during school, no marks that can be seen by others, no physical violence, nothing overtly sexual, and either can end the relationship or refuse at any time.