Without being married, or signing a piece of paper, there's no one in my life who could make medical or financial decisions if I were incapacitated. Maybe you have other relatives who could by default (and that you trust), but I don't. Having spent the last few months dealing with medical issues, this is more important to me than I had ever anticipated.

I see your point at a basic level, but I feel like you're missing out on details that really make an impact in the long run of the marriage itself. Maybe they aren't important to you today, but someday they may be.

Maybe whatever you do can be called a partnership or something else if you're more comfortable with that.