Contemporary debates about sexual ethics are dominated by a consent-oriented approach—consensualism. This position well explains the immorality of such acts as rape, pedophilia, bestiality and necrophilia. However, consensualism faces difficulties when it comes to adultery or HIV transmission. This article analyzes such unacquired moral obligations not to engage in consensual sex. A new natural law approach is proposed to explain and justify these obligations. This position places central importance in the evaluation of sexual acts on whether they are aimed at (...) achieving some common good and whether they harm some other good. As will be shown, the new natural law overcomes the difficulties of consensualism and grounds unacquired obligations in sexual ethics. And if this approach is combined with a pluralistic view of the goods that are realized in sex, then the new natural law does not allow the conservative conclusions about sexual morality inherent in the old natural law theory. (shrink)