Kantian Ethics - The key question here is where do the formulations of the Categorical Imperative leave us in regards to extramarital sex? For Kant, marriage involves a promise of fidelity, so extramarital sex is wrong as a form of promise-breaking, which cannot be universalised but also in that it often treats the other individual in the adulterous relationship as a means to an end of pleasure (as adultery is often for sexual gratification/out of lust), hence violating the 2nd Formulation of the Categorical Imperative. Marriage involves a contractual promise of life-long commitment. For Kant, we have a perfect rational duty to not break promises and to fulfil our terms of our contractual duty, so divorce also seems wrong. Furthermore, for Kant, Marriage involves giving ourselves completely to another person, becoming a 'unity of wills', so that one is not merely used as a means to an end in sex (sex objectives us and marriage avoids this objectification as it unifies the couple). To have rights to someone sexually therefore, you have the right to the whole person since a person is an absolute unity according to Kant. You cannot have this right without giving them the right to your own person and as a result, through adultery, you are giving away yourself and your partner implicitly to another without the other partner being away and hence not acting autonomously, hence showing that extramarital sex is wrong as it is not autonomously entered into by the other person in the marriage.