- a mutual decision to allow such things is similar in that respect, then both parties are guilty of the same thing: Breaking the marriage agreement. I'm just asking you whether this principle is an ad-hoc thing that you only apply in the case of marriages, or whether you think it should apply to contract law as well. Makes sense to be consistent, right?
Not really, marriage is not a contract in that respect see; R v R  1 A.C. 599, House of Lords.
This case is about marital rape but the "marital contract" is discussed in obiter.
People don't get divorced because of a broken agreement they get divorced because of irreconcilable differences that may be evidenced by
adultery, abandonment etc...
Also, in contract law if both parties agree to a change in the contract neither is in breach see - battle of the forms etc.