Immigration status for your partner. Saving money on tax returns. Medical rights, employment benefits, legal rights, burial rights - all things that otherwise go to the family of origin, i.e. the homophobic parents who will pretend you don’t exist. We went through this time and again during the AIDS crisis where the family would show up days after someone died, kick out the boyfriend, take all the dead guy’s stuff and pretend he actually died of prostate cancer or something. If you name wasn’t on the lease - and it couldn’t be because you weren’t legally a couple - then you got the locks changed on you and were out on the street. Room mates have no rights. Did you own property? - well the family inherited everything because you didn’t legally exist. We held funerals in bars, there was nowhere else to go.
Marriage is a bundle of rights and responsibilities that would cost thousands of dollars to legally replicate, and even then most of the wills and powers of attorney can be overturned by the ‘real’ family. We need legal marriage to protect our relationships.