I am thinking now of the couple who opened up their relationship after one of them became too disabled for regular physical intimacy.

There's probably been more than one.

Is that still a disaster? If everyone involved is adults and in agreement and it is sustained over an extended period of time.

And if people call each other "husband" or "wife" they're married. That's how marriage has worked for most of human history. You decide you're married, that's it. We add a lot of fripperies to it, but even in the UK "common law" marriage, that is to say marriage without a ceremony is still recognized.

You're coming to this from a narrow perspective and in what way, in any case, does the wonderful variety of human perspectives affect you? Why should the definition of marriage you are using win out?

The definition of marriage, what marriage means, how you enter into it and with whom change with time and place. We aren't geese, we aren't naturally wired to have a monogamous relationship with the person we reproduce with. Oh wait, geese aren't either (many monogamous birds cheat and geese practice surrogacy!)