No-one is saying anyone should get married. But I agree that those who aren't seem curiously defensive about it. Who said anything about bended knees? And why are people ignoring the point about the other important legal issues which cannot be dealt with?
All I and others are doing is pointing out the legal position, and sympathising with the fact that OP is in a a relationship where one partner is not respecting the other's wishes, and it's making her unhappy. Those who aren't married seem incapable of acknowledging that she is entitled to feel any anxiety, which is frankly baffling.
The whole next of kin issue is a bit of a red herring tbh, yes there are ways of dealing with it but unless you actually do something about it, it could be a problem. Like everything else!
The OnePlusOne website gives an an excellent guide, please read it. As a broad summary, it tells us:
There a BIG differences in your position with regard to taxes, pensions, separation, leaving a will, and inheritance.
There are FEW differences in your position with regard to banking, debts, benefits and child support.
There are SOME differences with regard to immigration.
All you have to do is click on the headings and read:
www.oneplusone.org.uk/marriedornot/LegalOverview.htm
The whole point is surely that if you are married then the next of kin issue, along with all others, is automatically dealt with. And some cannot be dealt with by seeing a solicitor, even if you want to go down the route of chasing several pieces of paper rather than one.