I have two DCs with DP. We aren't married and aren't likely to get married.
I saw a solicitor after our first DC was born, to see how the land lay legally. She said the quickest way to get maximum legal protection in terms of money and property would be to get married.
However, if you want to remain unmarried, there are ways you can replicate most (not all, but most) of the legal protection via other remedies. So DP and I did the following:
Own the house 50/50. We actually own as tenants in common because some family members leant us significant sums of money towards buying the house and we wanted to protect their contributions. But ideally you'd own as joint tenants.
Each made wills with really explicit instructions about who gets what inheritance if the other dies
Took out life insurance policies and named each other as the beneficiary
Willed our pensions to one another (although, because we're not married I wouldn't be allowed to claim the state widows' pension if DP died)
I think those were the main things.
One thing I did say to DP was that if, for any reason, I had to give up work or significantly reduce my hours/earning potential, then at that point I would want to be married. Because then, in the event of a split, the hit to my career would be recognised in any settlement. This is of course also still true and applicable if the roles are reversed.
IMO people romanticise marriage too much. It's a legal contract binding the individuals into agreements about property and money.
If you want to be together and have children, then be together and have children. Just make sure you hold onto a legal claim to your house and your money. Whether via marriage or some other means.