In the US at least:
What if they get seriously injured? How will you have rights about their care? To go and see them? What if you have great insurance, but they don’t?
Marriage has tons of benefits over being unmarried, right or wrong. Plus, I enjoyed getting married. I also enjoy being married. Life is hard. My wife always has my back.
Ask the legislature. Some of those you can get other ways, but not all of them. Getting married does them all.