A relationship is like a moral contract. Traditionally, the contract is that you should sexually satisfy your partner in exchange for their fidelity.
Of course, in civilized countries, you are now free to choose another agreement, including "fuck whoever you want" to slave contracts if that's your kink. None of them are legally binding since you are free to do what you want with your body, including "cheating".
There are a few remnant with regards to marriage. For example in France, both cheating and not satisfying your partner are cause for divorce. It is mostly symbolic though, "winning" the case doesn't give you much rights.
Of course, in civilized countries, you are now free to choose another agreement, including "fuck whoever you want" to slave contracts if that's your kink. None of them are legally binding since you are free to do what you want with your body, including "cheating".
There are a few remnant with regards to marriage. For example in France, both cheating and not satisfying your partner are cause for divorce. It is mostly symbolic though, "winning" the case doesn't give you much rights.