Married couples have the possibility of maturing and evolving, but cohabitating couples typically succumb to relational inertia. Without the foundation of marriage vows and shared selflessness, they cannot build anything stable. They live “day to day,” and have to shut out the possibility of planning for the future because that future may not exist, and, theoretically, they should have everything they want already. …
People who cohabitate believe they will have the best of both the single and married life. They believe that they will have the freedom of single people, paying their own way, leading their own life, and having the option to leave, while enjoying the stability of married people, having steady company each night. In reality, they usually have the opposite: they are not free, and it is not stable. This is because people who have decided to live together cannot simply break up as though they were only dating. Cohabitation is commitment without commitment, and people invest a great deal of themselves (their time, their emotions, and their money) into a relationship when moving in together—even if they say and imagine otherwise. After a few months, or a few years, it is far easier to keep things intact than breaking up a bad relationship, so nothing changes.
Moreover, people who cohabitate for so long have increasingly fewer options available to them once they break up. Instead of dating and meeting different people in their 20s, they loafed around with a subpar mate and let the best years of their life pass them by. Once they seriously look for a spouse in their 30s, they will have to tote around some heavy personal baggage.