To be a little bit sad I can't ever marry

This is crazy. You’ve been told over and over, you CAN protect your assets.
You are choosing not to marry and then asking for sympathy.
Yes, YABU to feel sad about this, because it’s not true. If you choose not to marry, it’s on you, and you are choosing to be sad. Fill your boots.

Have you actually looked into the option to do a will "in contemplation of marriage"?

Lock the assets in a trust, sign them over to DC now or sell everything and sail into the sunset. You’ve been given a million examples of how and why you’re being ridiculous. If you want to marry, do it. If you want to protect your children’s “Inheritance” first, go right ahead. There are plenty ways to do it.

If you want to choose your assets over the relationship you want because you CBA to do paperwork, then that's entirely on you.

Or, see a financial adviser and a solicitor, take their advice, do the paperwork and do what makes you happy.

A PP pointed out that there are disadvantages to not marrying, and they're not negligible. Google it - "unmarried couples generally have fewer legal rights and protections compared to married couples, particularly in matters of property, inheritance, and healthcare decisions."

Literally every penny you spend ‘reduces DCs inheritance’. That doesn’t mean you shouldn’t spend it. It’s your life to live now, not theirs to control because of what happens when you’re gone. I really think you are being massively unreasonable not to have the relationship you want, for the sake of possible future complications for them. You can work out what’s fair for them and for DP, get it in writing, and still move in together. I honestly think you are being ridiculous. Ask your children if they would genuinely want you not to have this relationship for the sake of their inheritance. Which, frankly, might end up all being spent on care homes anyway.

Life is for living.

I completely understand OP, this is one reason why I don't ever want to marry (even though I'd love to).

If anything happened to DH or between us that meant we were no longer together, I would never bring another man into DDs life and everything DH and I have worked for would be hers.

But not marrying again, not living with another man, that would be a choice I made. It is a won't not a can't and that's the same for OP. She won't marry or live with this man and she's complaining it's sad that she can't.

She can. She's choosing not to. And it's a perfectly fine choice, the same one I would make. She shouldn't therefore be acting all woe is me like it's being forced on her.

I think you have a very sensible and mature approach to relationships in that you are putting your DC first by not marrying and that's the right thing to do. Many people don't.

Your friend and her siblings sound like jerks.

She probably has to worry less about their security now that they're adults but I'd be damned if remarrying meant my childrens (adult or otherwise) inheritance was going to.be affected

So I took out the policy when I was 40 and I have calculated that over the lifetime of the policy. I will pay £34,000 for £1 million worth of cover. That doesn’t increase within inflation so it’s 1 million in today’s money.
The amount that I pay doesn’t change as I age so it’s not a case of the insurers making any decisions they are as committed as I am and as long as I make the insurance policy payments the cover remains in place no matter what happens to me cancer strokes anything else that might come along but doesn’t finish me off they have to continue to pay out.
But equally the kids are building their own assets at the same time so whatever it equates to whether it’s a public lunch or a house deposit it’s instantaneous.

I’m not sure actually. I think it covers me until I’m 85 and don’t get me wrong. It’s expensive and if I die at 86 the whole thing has been in vain but that seems unlikely.

YABU and being ridiculous. Get married and enjoy your life.

See an excellent solicitor beforehand, draw up a watertight will and put your house or assets in a trust for your DC.

The only reason not to do it is if he’s genuinely destitute and you’re very wealthy, but even then there are workarounds. If you’re sure he’s not a free loader just enjoy yourself and marry.

So what you're saying is that the first parent to die should have not left it all to their spouse in case they went on to remarry and didn't do enough to protect their children.

Well, yes. But that's precisely what a lot of people don't think to do.

In France the law requires you to do this, because it considers that sideways disinheritance is unfair. It does mean you can't disinherit an estranged child, but no system is perfect.

You can marry for goodness sake, just get a solicitor and a concrete plan for your assets that protects who/what you want it to. It’s not that hard, drama queen!

Just have a wedding/commitment ceremony without the legal document. Husband and wife in name only, on your terms. Legally “partners” but to you both it is a marriage on your own terms. Outlines those terms beforehand, so you know what you’re agreeing to. Romantic and personal and not involving the government.

Life is exactly what you make of it. If you want something there is always a way for you to make it your own.

Nor Pyramus and Thisbe.

Well it depends on the situation you're both in.

If you're both financially independent, then write wills so that everything goes to the people you want it to (you, your DC and him whoever he wants, but not you).

If he isn't financially independent, then think about what is best for everyone. Could you split what's left in your will? You could put your home into Tenants in Common, but with new husband only having a 10% (or whatever you want) share. If you die first, your share goes into trust for DC but new husband still has the home to live in.

Pensions etc you could keep to yourselves. Or leave the house to the kids and pensions to new husband.

You'll find that your kids are much more worried about mum being happy in her life than whether she has any money left to leave them. So do what is best for your life not what's going to be better for others after you die.

Being married IMO means nothing anyway!

I spent around a dozen years following the legal battles for same-sex marriage in the UK and the US. Before laws changed, those were couples who really and truly could never marry. You, on the other hand, just don’t want to deal with sorting out some complications about wills and inheritance.