"What happens if both work during regular office hours (as in my marriage)?"
OP specifically mentioned this is not the scenario we are discussing.
"If you have very young kids, it's impossible to have a distraction free work environment at home unless you find someone to take care of the kids and enforce the no distractions rule."
Again, neither OP or I are arguing against this.
"Both parents are caregivers in a marriage, there's no single caregiver."
Again, this feels like an uncharitable reading of my statement. Of course there are two caregivers but often there is one person in charge of the kids, hence my caregiver (singular) statement. Which partner it is, is irrelevant.
"People often assume, somewhat unfairly, that this person should be the mother."
How is this relevant to this post?