There are so many different variables to this. It also depends on the sector you work in and the market rates for the jobs that you do.
DH and I are the same school year age, and I am that much-despised creature, the highly educated woman who resigned from my job to be a SAHM for many years. For two key reasons - I didn't feel I was giving enough to the job, and I actually really enjoyed being at home with DC. DH is a lovely, loving parent but would not have wanted to be home full time. I did. I'm sure there's a huge amount of social conditioning in there somewhere but it's the reality of our situation.
I wanted to be home with the DC.
Three DC and many SAHM years later, I'm now working again (and loving it) but I earn literally a fraction of what DH does. He carried on his career and earns a lot, I now work PT in a school which is lovely and fits well around the DC but earns me very little.
So - shock, horror, yuck, etc, yes, I prove the 'where does it all go wrong?' statistic.
BUT - the reality is that even if we had not had children, even if I had carried on with 100% focus on my career, with no distractions and no maternity leave breaks, I would not be earning what DH is now. I'd still be earning around 1/3 of his salary, as I was when we first met. For the simple reason that we picked careers with very different remuneration prospects. I was in HR when we met, DH in finance, there are lots of parallels between what we both used to do (supporting business partners, lots of pressure/demands from said business partners etc) but the reality is that accountancy pays more than HR and DH was outstripping my earnings even when we were in our twenties and child free. Nothing to do with being a man and everything to do with being a qualified accountant rather than an HR professional.
It's just different career choices and different skills sets in our case.
But I do agree that it does 'go wrong' for so many and I also think it's not just because society dictates it. I just wanted to stay home with my DC because I genuinely enjoyed it and wanted to. I'm not sure how you get around that fundamental instinct.