A couple things that may help with this chapter. One of the key questions that comes to mind is, why do we see having children as a blessing all over in Scripture and yet it is a cause for uncleanness here? For starters, we can’t drop that context. Children are indeed a blessing and women are created equally in the image and likeness of God. Any assertion in how to interpret this section that diminishes either of those two things is unsatisfactory. Also, note that the very act of intercourse, the nuts and bolts of creating this blessing, make both participants unclean. Broad point is, it’s irresponsible to try and render a moral judgment on what’s going on here without the context that surrounds it.
Further, we should be clear that being pregnant is not the issue in play here. Neither is the birth itself. It’s what is occurring after the birth, namely, the loss of blood. Similar to the lady’s monthly cycle, there’s a time after childbirth where there is continued bleeding. It’s heavier at first (and bright red because it’s fresh) and then continues to get both darker in color and lighter in volume over time until it ceases. This bleeding can last between 2-6 weeks (normally). As we’ll read later on, the lady is contagiously unclean for the week during her menstrual cycle. Anything that makes someone unclean beyond a week generally means they aren’t “contagious” in uncleanness after the week but it also means it will require a sacrifice to deal with (purification for the altar since there was uncleanness in the camp and then the burnt offering for forgiveness of sin.)
But why does this loss of blood make someone unclean? Perhaps it’s because loss of blood can lead to death, the antithesis of a “normal” life (consider our senses of “normal” or “whole/complete” from the last chapter). The women’s cycle is intended, in its normal state, to produce a child. In months where it doesn’t do that, it creates a state of uncleanness. Does God realize it’s a normal body function? Of course. Is it identified as a sin or something the women is doing incorrectly? Of course not. Regardless, it is something that makes her unclean and must be reacted to (not dissimilar to the unprovoked emissions from a fella during the night. He didn’t cause the thing, it just happened, and it still makes him unclean).
The question of why it’s a longer time when a girl is born vs. when a boy is born is a bit harder to nail down. Some believe that the circumcision of the boy has the impact of reducing the time of uncleanness for the mom. There is also some evidence to indicate that the time of bleeding of the mom tends to be longer when a girl is born then when a boy is born, although i wouldn’t lean on that without some studious inquiry on how repeatable and persistent that medical situation actually is.