@Nou Wives are to "pay the marital debt" because that is their duty (and pleasure) to their husband, not in order to obtain resources. Husbands, likewise, are to have sex with their wives for the exact same reason. Scripture does not present marriage as a transactional affair.
Husbands *also* provide, but not in order to purchase sex. It is, per Ephesians 5, to be done without conditions per the Agape love charged to them.