In theory you can replicate a lot of the legal protection of marriage through separate contracts.
You can get powers of attorney, both medical and financial, to ensure you are the person making decisions for your OH.
You can ensure all bank accounts are joint, and that all insurance and pension payouts go to your OH (nowadays most let you nominate the beneficiary). Ditto any death in service payments from your workplace.
You can draw up and sign a cohabitation agreement with a lawyer, which is sort of like a prenup and makes it very clear who owns what and what financial responsibilities you have towards each other if you split, particularly if children are involved.
You can ensure your will makes the same very clear.
But the above costs money. A lot more money than a simple registry service with two strangers off the street as witnesses.
Also, at the moment inheritance tax has different thresholds for non-married couples, so you could lose out there by not being married. And you aren't eligible for a widows/widowers pension. And if you separate, the partner who earns more doesn't owe financial support for the other parent if there are any children, which they do if you are married.
We're not married by choice. We have all the above; inheritance tax is currently not an issue. But if we didn't know lawyers who did this for us for free, I probably would have insisted we married (against both of our wills) for the legal protection.
If none of that applies to you, you trust each others family to know you are each others next of kin, and the finances aren't a worry, then you're fine.