The point is that a stay at home parent has more value than a single profession because two "labour units", if we must think of things this way to make it clear

The point is that a stay at home parent has more value than a single profession because two "labour units", if we must think of things this way to make it clear, are made less effective by having to practice their profession as well as maintain their home and raise their children. The argument in favour of a stay at home parent is that two working parents are made so ineffective that they are <50% capable as they were before forming a household so instead of 2 you now have less than 1 and in order to fix this one of them takes on all of the work of maintaining a household and raising the children while the other focuses their energies on their profession outside the home. This results in at least 1 full unit of labour but it may actually be greater than the average since the individual working outside the home may now be performing at a higher level since they don't even have to maintain the homemaking that a single person does. On top of that they gain the benefits of being in a stable relationship with a spouse and children and on top of that their relationship to both may be even better than other couples where both work. The person managing the household may even have a small participation in the labour force as mentioned in the article so you can add that to the total.