Kant calls marriage a contract in which two people mutually consent to grant each other reciprocal rights, and so it is not about one person using or abusing another as there are equal rights between both parties (the contract benefits both parties equally) and hence, in this scenario, sex may seen as duty-bound, as a way to reproduce for the continuation of the species (it would be irrational to not reproduce/want to continue the species) but also to be faithful and satisfy your partner. Kant did not see sex within marriage as wrong, and the purpose of marriage, as he sees it, is NOT procreation, as in Natural Law, but the union of two people of different sexes. Sex is morally permissible within the context of a heterosexual, lifelong and monogamous marriage. In marriage, husband and wife are equal partners (as both are autonomous and rational moral agents) and sexual enjoyment is a right to be permitted within the partnership. Hence, as a component of a fruitful marriage, one can argue that as sex strengthens the union between partners (and from a Catholic POV, is an essential component of a valid marriage), sex may be seen as a duty/contractually obligated in a marriage, leading to issues about how if sex is duty bound within a marriage, there is a moral pressure to engage in sexual activity and any sexual assault/rape within a marriage can be justified as merely following one's marital duty