Why this discrepancy in the length of time an Israelite woman would be considered unclean?
At first read it seems rather sexist.
What did being unclean imply?
During the time she was physically unclean, the woman was not to have sexual relations with her husband. This promoted self-discipline and hygiene in the Israelite culture. During the next 33 days, in the case of a baby boy, or 66 days, in the case of a baby girl, the mother was ceremonially unclean, which meant she was not to come into contact with holy objects (Lev. 12:4,5; Insight on the Scriptures, vol. ii, “Mother,” par. 3).
Why was the new mother “unclean” to begin with?
The research explains that because of the imperfect state of humanity, the reproductive organs which were once meant to pass perfect life, now actually pass the “inherited effects of sin,” that is, illness and death (Watchtower, 5/15/2004, p. 23; Ps. 51:5; Rom. 5:12).
This ‘passing on of sin’ required some atonement, which is why God asked for a burnt offering and a sin offering, (Lev. 12:6).
These sacrifices foreshadowed Christ’s sacrifice which would atone for all of repentant humanity’s sins (Gal. 3:24; Heb. 9:13,14; 10:3,4).
To put this into a more familiar context, let us remember that even Mary recognized her own sinful state when she and Joseph presented baby Jesus at the temple and offered a humble sacrifice (Luke 2:22-24).
Israelite birth customs drew attention to our lamentable condition of being born in sin as opposed to perfection, and therefor highlighted the need for a savior from this state, the need for the expected Messiah.
So why the discrepancy in the length of “unclean” time between the sexes?
In family life, a woman is to subject to her husband’s role as head of the household (Eph. 5:22,23).
“Thus, from birth, the Law distinguished between male and female, assigning to the latter a subordinate position,” (Insight on the Scriptures, vol. i, “Clean,” par. 11).
From this we can conclude that the time needed for a mother to become physically and ceremonially clean after giving birth to a girl served to remind society that not only were they all sinners, but females should subject to their husbands/fathers within family life.