It's the gender-neutral, unpaid-work-respecting equivalent of a "stay-at-home mom" or "housewife"

If a couple decides they want four kids, the numbers can work out, for 10 years or so - especially if childcare is expensive.

Yup - the "numbers" can work out with even one kid, particularly if your equations value time spent with the child.

All the more reason to be a full-time parent. You get a return on your investment extremely fast. When you're old, you have adult kids that care about you, and hopefully grandchildren which are basically just children but free because you're not the primary caretaker anymore. All the fun with much less fuss.

I presume GP doesn't mean someone with no other identity, just someone who is doing it instead of a FT paid role. I could be a full-time plumber and still have an identity outside of plumbing