I out earned my ex and so am a woman who would have been better off outside marriage, but I am still very much the exception, my lawyer said she sees few of these cases, though they are growing.
The main issue for many women who are equal before children is that it can do a number on your earnings and savings. Maybe you're earning the same and equally ambitious, but then you get pregnant. You don't go for that promotion because you're due off on mat leave in 3 months. He does, and gets it, or if he's self-employed, he puts in more hours to help support his family. You are on maternity leave and run down your savings a bit, even if you share bills, because you feel guilty using 'his' money for haircuts or new clothes, or non-essential but pretty things for the nursery. You go back to work but only 4 days. Another promotion is up for grabs, but it will mean more hours and monthly travel so you leave it for now. Or maybe you go for it, but it goes to the person who has 'shown commitment' while you were away on leave. Then maybe there's a second pregnancy and it repeats again but this time it's hardly worth going back because of childcare fees so you drop more hours. Your eldest starts school and somehow you're always the one they call when a child is sick, and also your partners' career is now better earning than yours so it makes sense that you do the dropoffs and take the hits on unpaid leave. You minimise your pension contributions to maximise your lower take home salary. His is increasing, in line with his pay. You spend a lot of time and some money doing up the house in his name to make it nicer to live in or get better rent.
Then he's gone.
If it's a tragic accident, your career is now going to be a whole lot harder to resuscitate, and you'll be paying IHT on his house.
If (more likely) there's another woman on the scene, he heads off with his house, his savings, his pension. He should be liable for CMS but many women on here can tell you how badly that works for some, especially with self-employed people who can manipulate earning figures.
If you were married, you would likely have had a share of his house, of his pension, potentially even spousal maintenance if you'd given up work. None of us ever think this will happen to us, we imagine that our careers will continue flexibly and out partners won't tire of either is or of family.life and no-one will.get sick. But these things do happen, and marriage helps you prepare.