The way courts handle the affairs of unmarried people is to figure out what their relationship most clearly resembles. If it resembles a marriage they’ll handle it that way, if it resembles roommates they’ll handle it that way. And if the couple has a preferred method for the government to deal with them they’re best filling out the paperwork.
You WILL recognize theirs whether you like it or not. If you want your spouse to get you stuff when you die then there’s some sort of paperwork you need to handle. You can write it up in a complex contract that isn’t a marriage license, and get it in your will, and hopefully you dotted all the i’s and crossed all the t’s to get it all right, and hopefully you don’t have any estranged children, and neither of you has any kind of previous not quite a spouse, or even a business partner that could stick their nose in, and sure hope if you have surviving parents or siblings they like your not legal spouse and won’t interfere. Or you can just file a marriage license.
Nothing is unchallengeable, certainly not marriage. Many a second wife has found themselves in court with the deceased husbands business associates, children, ex-wife, etc.
A legal contract is no less binding, and by nature of its specificity, is often more binding than general inheritance/divorce law.