Ummm-- nonsense? Lots of people have one person, above all others, that they cherish. And for most of life, it doesn't really matter that this is the case (but like, for most of life, my hobby doesn't actually much matter to other people). But sometimes, that one person I cherish really does have extra power. Who gets to visit me in the hospital (answer: the one I cherish does!) Who gets my kids if I die (answer: the one I cherish does) Who gets my stuff if I don't have a will? (answer: the one I cherish does).
And hey, isn't it handy that there's a super-simple, cheap way to tell who I cherish: it's the person holding the marriage certificate! So an entire mass of horrible, messy, expensive problems becomes simple and clear.
Oh, and it also turns out that there's a nasty problem with the way that humans procreate: it's really long term, only one gender can do a bunch of the hard work. And often people who cherish each other have a commitment that one will do more of the looking after kids and the other more of the earning money. And because it's two people that work like one unit, it makes sense to fiddle the tax codes a bit so that it's more or less fair. (Like everything in tax code, there's always corner cases)