I believe that craig1f is claiming that if most women did not work, wages would be higher for those women that do work (due to reduced labor supply), so increased labor participation for women has lowered the opportunity cost compared to a hypothetical world in which women work.
So back when being a stay at home mom was the norm, it was hard, but now that working moms are the norm, being a stay at home mom is easy? I don't think that checks out.
While being stay at home was the norm, around 33.9% of women in 1950 worked and it continued to go up. That is not completely negligible number. (The number for males was closer to 86.4%)
You claim to be disagreeing but I think you are agreeing with that comment. By referring to "opportunity cost", the author means precisely that when a woman works it is harder to have a large family. Not having a job is one opportunity cost of children.