It always seemed to me that instead of fighting over definition of a marriage, governments should just generalize to some kind of civil union to allow broader scope of living arrangements to benefit from recognition and protection. Two (or more) people sharing livelihood doesn't need to be based on sex or kinship. Adoption is just a crude workaround for the lack of the right civil law abstraction.
There are a lot of legal and healthcare reasons to have the government recognize your partner. Things like:
- Having someone able to make medical decisions when you're incapacitated
- Insurance coverage and other entitlements from a partner's employee benefits
- Power of attorney
- Social security entitlements
- In some jurisdictions, to inherit property after death instead of it going to children or even the state
- For people in the military, a variety of valuable benefits that can mean the difference between living in a shared barracks room and living in a house off-base