What MarkTwainiac seems to be saying is that the disproportionate burden placed on a woman when having biological children rather than adopting pales in comparison to the 18+ year long commitment to child rearing by both parents.

Imagine a situation where the father does 55% of the childrearing over 18 years, while the mother does 45% of it. He's just slightly more into the kid, and slightly more involved. Over 18 years he will do the equivalent of 9.9 years (9 years and 11 months) of parenting and she will do 8.1 years (8 years and 1 month). Even if you add pregnancy to her share, in terms of total time invested he's still doing over a year more parenting that she is.

Of course its slightly different when talking about physical investment of pregnancy and birth vs emotional and time investment, but then there are plenty of women who have extremely easy (even pleasant) pregnancy and birth, in which case you almost shouldn't count it at all. My mother has said she never in her life felt healthier than when she was pregnant.

But I believe that's what MarkTwainiac said when she said you're reducing 'having children' to 'making babies' and therefore ignoring the biggest part of being parents, so your hypothetical isn't really valid.

I think you should also consider that temperament is at least partially inherited. Having a baby/child/teengager who slots right into your family dynamic and naturally bonds with you from the moment of birth is likely to be a lot less work than adopting a child, building a bond with them over months or years (which might not ever totally form), and working through and abandonment or other issues they might have as a result of being adopted. So even when you only consider the strain on the mother, adoption is not the silver bullet you might think it is.