My family have naff all money, so I'm not getting protective about a future inheritance - I have no agenda here.
To me, it does not sit right that inheritance becomes shared money.
In a normal world, if you are close enough to someone for them to leave you money, you must have an emotional bond, and you must rather have them in your life, alive than a pot of money. The cash is a bit like compensation - it's like when you lose a limb and insurance pays out. The cash can't put your life back to where you were before, some things are irreplaceable, but it's intended to make your life a bit less hard. Not someone else's - it's about your pain and suffering.
I mean, if you lose a parent, that's a special kind of grief. I don't think many people would feel the same way about losing an in-law.
Inheritance isn't earned per se, it goes to you because of a relationship you had, that no one else could have had. You built that relationship. It's your loss. It's your grief. And the person who leaves you the inheritance leaves it to you, not your partner.
With employment income, you earn that yourself, so if you want to share that with your partner, it's entirely your money to decide what to do with. (By marrying, you've decided to share it - that's what that contract does.) With inheritance, someone chose to leave that to you, and if you let someone else just take it, it feels like you've trampled over their wishes. (Legally, I think in many cases if you're married, your partner has a claim on it, but morally, I think that's wrong.)
I mean, if you want to share your inheritance, that's fine, but I think expecting to just have half of a partner's inheritance is poor taste. They've just lost someone special to them - the money is supposed to make up in some way for that loss. The source of the money is your partner's pain.
(But I wouldn't get married anyway, because at this point in my life, it makes zero sense. I would lose far rights more than I would gain. As has already been said, things get more complicated the older you get.)