Maybe nothing has changed with regards to the history of marriage
According to Stephanie Coontz, author of Marriage, a History, the current model for marriage involves more opportunities for fulfillment. Both men and women can work outside the home, and love is on the table more frequently. But maybe the total proportion of adult heterosexuals who are fulfilled in relationships, marriage or otherwise, is the same. This may be inherent in the differing mating strategies of the sexes, which hasn't changed. Economics, politics, and reproduction are always in flux, but the same basic pollinating male vs flowering female is fixed in our biology.