Married couples also have legal and implied rights that non-married partners do not have, especially if/when one of the people is seriously injured. There can also be financial benefits for taxes, especially if you have dependents.

If my partner is in the ICU and we're married, I can sit with them and have a say in the care they receive. If we're not married, I often can't sit with them, nor can I get most details about how they're actually doing (HIPPA laws; you can get around this if you put waivers in place before needed), nor have any influence on their care.