Why is adultery a sin?
From what I read and hear, many Christian answers have to do solely with the breaking of a contract. Marriage is a contractual arrangement in which both parties have certain rights, privileges, and responsibilities. Among those responsibilities is sexual exclusivity with the marriage partner. Breaking that contract harms the other person and is therefore a violation of the law of love.
The sin is entirely explained by the harm caused another person.
One problem with such an argument is that lots of marital arrangements suddenly become kosher: open marriages, spouse swapping, anything else where both parties agree it does not violate “the contract.” So long as the parties involved agree to the terms, it is just fine. No harm, no foul.
Of course, for Christians the Ten Commandments and the teaching of Christ make adultery about much more than human obligations. Adultery is a sin because it is rebellion against God. It is rejection of Christ’s lordship.
We do not have to explain the sin in terms of human harm — although we can in this case. Adultery is a sin because God has declared it so.
Is there a better answer than this?