Marriage isn't losing its relevance. It might be less necessary for some couples, and it might be more necessary for others, for various reasons.
Some people think getting married isn't a requirement for a healthy, long-term, committed relationship, that it's just a piece of paper.
Some people think getting married is a right that should be afforded to everyone, and in whatever situation they might be in, they want to marry as a symbol of their healthy, long-term, committed relationship.
For many people, getting married is still an almost innate inclination, the natural next-step to show their love for another person. I would say for most people who get married, or who want to get married, it's for that reason. For love, for unity, for the tangible evidence of their commitment to their spouse.