Since both parents now work, often with lengthy commutes, there isn't a whole lot of person-hours left to spend with the children. That means people have fewer of them.
When the kids get older and it's time to send them to university, well, that's something which has gone backwards in affordability. The cost of a university degree has shot through the roof. How could anyone afford to send 3 kids to university these days, let alone 8?
Why do you say especially the woman? Not trying to pick a fight, I'm genuinely curious.
Kids have turned from a resource like they were to farm families to an expense and an increasing one at that - and I don't mean in a "kids these days" sort of way - the world changed and not people. Which leaves children to be had by those who want to have them for the sake of having them - some sort of enjoyment or sense of obligation.
People want to buy more stuff? Hardly a requirement of years gone by.
People want more free time? Doubtful, otherwise people would work less and buy less stuff - which we don't see. People keep getting more productive, but they just funnel that into a more expensive lifestyle rather than cutting back on work.
I don't think the reasons are simple (or even the same between people) but I don't think it's dominated by resource constraints.
I know software engineers with no children and school teachers with four. Very different lifestyles, but all middle class. And I don't think resource constraints factored heavily into their decision making.