In a perfect and equal world where pregnancy & birth is perfectly straightforward, baby is born totally healthy, no learning difficulties at play etc, both parents earn the same amount, pay the same amount, own 50% each of all assets, and do totally equal amounts of parenting, there really isn’t a huge need to marry. But how often is that the case?
Pregnancy & birth leaves some women unable to work again or unable to do the same work/amount of work again, suddenly they’re worse off financially, unable to save or build pension, so unmarried means dad can save up and then walk away with everything and mum is left with very little.
A disabled child or child with ND, or even not ND, can struggle in school. If a child can’t attend school or nursery, someone needs to be there, so again, someone (usually mum) has to stop working to be carer.
Even where all is perfectly fine and child is in school, someone has to be available to do drop off, pick up, sick days, inset days, parents evenings- if both parents are going to split these equally and it doesn’t impact earning then great, no need to marry. But reality is one parent, usually mum, has to reduce hours or pass on promotions which require less flexibility/more travel/more responsibility because someone has to be available for the kids. Someone has to have the job where you can go “actually I need to leave, child is ill/inset day/parents evening”, the other parent without those restrictions can progress in their career, can do the late nights, the travel, the networking, without having to consider childcare. So after 10 years of that actually dad is earning £30k more and mum is either on the same as she was or less if had to reduce hours, dad couldn’t have moved up without mum picking up the brunt of the childcare, but dad could walk away in 20 years with his big savings & pension. Even where things may look equal with a 3 year old, they won’t necessarily look equal in 15 years time when one person has had to shape their career around flexibility, being available when needed, being a bit unreliable as an employee due to last min sick days for child, not being the worker first in & last out of the office because they are at the school gates, not being at the networking dinner because they are getting the kids home & fed etc.
It’s a bigger picture thing and it’s one where there are so many changing variables it’s impossible to say early on that you’re better off for being unmarried- it’s almost never the case.