One of the biggest features of a marriage is that the husband and wife live together. The same goes for people who are cohabiting. While people who are cohabiting may not have made a lifelong commitment to one another (although it's not out of the question), people who are married have. However, both kinds of couples do live together and therefore spend most of their time together.
Both married people and cohabiting people are in serious relationships. People don't generally decide to cohabit with someone they are just casually dating. Rather, couples who live together are usually fairly committed to one another. People who are married are extremely committed to one another but neither couple will be unsure of their relationship or in a relationship with someone they don't know very well.
While neither married nor cohabitating people necessarily have to completely pool their resources, both kinds of couples do have a financial stake in the relationship. At the very least, both will pay half of the rent or mortgage on a house or apartment. This means that both people in both kinds of relationships have something tangible to lose should the relationship fail. Basically, both kinds of relationships are higher-stakes than the relationships of people who don't live together.
Both married and cohabiting people are in intimate relationships. This essentially always includes some sexual intimacy; if there is no sexual intimacy in a married or cohabiting relationship, then it is not really a marriage or even a romantic relationship. While some people date without living together without having sex, couples who live together are almost always in sexual relationships, whether they're married or not.