The moral regeneration of mankind will only really commence, when the most fundamental of the social relations [marriage] is placed under the rule of equal justice, and when human beings learn to cultivate their strongest sympathy with an equal in rights and in cultivation.

Mill is arguing that genuine moral progress depends on reforming the most intimate and foundational social institution: marriage. As long as marriage is structured by legal and social inequality—especially the subordination of women—society trains people in domination and dependence at the very point where character and sympathy are most powerfully formed. By contrast, a marriage governed by “equal justice” would habituate individuals to treat the person closest to them as an equal in rights and education (“cultivation”), thereby deepening sympathy, mutual respect, and moral development. The claim links private relations to public ethics: equality at home becomes the seedbed for broader human “regeneration.”