[reuben_tate]

Define the boolean function c(p1, p2, A) as the result of the truthfulness of the proposition: "The action A is considered cheating in the relationship between p1 and p2."

If p1=p2, say both equal person X, then we can define c(X, X, A) as "In X's personal opinion, A is considered cheating."

define C(p1, p2) = {all A | c(p1, p2, A) = true}

Now, there are two extremes we can have. Either both people in the couple are lenient and allow their partner to do whatever where only the mutually agreed upon activities are deemed as cheating. In the other case, both people in the couple could be strict and abide by not only their own opinions of what is cheating but also by their partner's. Thus we can bound the set of activities that are deemed as cheating between a couple as follows:

(C(p1,p1) ∩ C(p2, p2)) ⊆ C(p1, p2) ⊆ (C(p1,p1) ∪ C(p2, p2))

Is it up to the couple themselves through proper communication to more precisely define the elements in C(p1, p2) (the things considered cheating between said couple.) Also, until you talk to your significant other, you have no clue what C(p2, p2) is (assuming you are p1) so it's best to talk to your partner about it so both of you can first have an understanding of each other's personal opinions and then after that, you can have a logical discussion to find a happy medium that satisfies both people in the couple.