^For the sake of the pedantically minded, of course a cheap registry office contractual marriage is cheaper than having a child! However, some couples with children still want the 'wedding' and not just the contract and why not?
Our ability to get married is not governed by a body clock, unlike our fertlity.^
So if having DC is going to leave you the more vulnerable partner financially, and marriage would offer you protection, why wouldn't you want to get the legals sorted quickly?
You can still have the contract AND the wedding at a later date!
I know loads of people who did that. They got legally married when starting TTC / when pregnant before the baby was born, with a view to having a wedding celebration later on, when they could afford it
Why would you want to sacrifice legal protection if you're in a situation where it would benefit you for the cost of £120 - you can still have the wedding later on.
Alternatively, you spend £800+ on a legal cohabitation agreement and hope you don't die before you're married and your kids can't access bereavement allowances.
If you would be financially disadvantaged by having DC in any way, and are in a situation where marriage would confer financial protections in the event of a separation, then I fundamentally don't get why you wouldn't put a cohabitation agreement in place to ensure at least some financial provision, if you're insistent that you don't want to get married.
Of course, a civil marriage is cheaper than a cohabitation agreement, but if you're adamant you don't want a marriage without a Big Wedding, then that's your choice.
If you have DC with your live-in partner @aranchini, do you have a cohabitation agreement in place, in addition to life insurance and wills?
If not, why not?