There are plenty of ways to show commitment besides getting married. For example, living together. Deciding to move into a place with one another offers a new level of commitment for a couple. This incorporates new responsibilities such as financial duties, contributing to household chores, and being with one another constantly.
There are plenty of times where a married individual will step out of their marriage and commit adultery. It is much easier to step out of a living situation than it is to file for a divorce. While marriage is the ultimate act of monogamy, it is not always the most sensible choice for some. Living with one another is an added layer of the relationship which tests the strength of the partners.