The thing is, a will trumps a marriage certificate anyway.
There are two conflicting issues here though - it depends if you are talking about division of assets in a split or a divorce. And its really easy to protect both people in event of either, with a little common sense.
First, don’t be a SAHP unless you are independently financially well off. It’s leaving you financially vulnerable regardless of your marital status.
Second, make sure your name is on the deeds of any property or tenancies. Then, you have legal rights to remain. Make it clear in your will what should happen to any assets from prior to marriage (I’m of the opinion that anything obtained prior to marriage is NOT a joint asset and I’d be leaving this to my children; however this doesn’t apply to us)
Next, have LPA to allow your partner (or anyone else - even a friend) to make medical decisions for you should you be unable to make them yourself. This is useful for married or unmarried couples.
LPA also allows any named person to make financial decisions for you.
Name someone on your pension as a beneficiary. This can be literally anyone you like.
Have a will in place with your wishes at to what happens if you die.
You are no better protected if you are married than if you take these steps in a long term relationship.
And to use the “wills can be changed” argument - of course they can. If I owned my property in my own name and I died; the property would go to whoever I named on my will. However, as the property is in both our names; my partner becomes sole owner.
The exact same is true if we were married. I could theoretically leave my house to, for example, my parents, and even if I married my partner, it would still be my parents house (unless my partner is also on the deeds)
Basically, a will is significantly more powerful than a marriage.