Leviticus 12 (ESV) seems to paint a pretty gender-biased view from our overly sensitive gender aware lives. In fact, it seems pretty anti-female. However, even the ESV Study Bible doesn’t say this, but actually aserts that since the text does not mention why, that it is too much inference. So, where does that leave us?
The mother is unclean (stated specifically) because of the blood that was a result of the birth. It is the sex of the baby that is crucial. Back a few generations, God commanded that a male baby be circumcised. That circumcision was to happen on the eighth day. For the sake of the continued adherence to the commandment of circumcision, the mother is not as unclean on the eighth day.
God did not command that girls be circumcised (a horrible and unjustifiable, in my Western opinion, mutilation of young girls that occurs today), but that boys must be. Therefore, it is my thought that the entire difference of the uncleanness of the mother due to gender is based not on gender per se, but on circumcision.
Circumcision was an outward sign of one’s membership in the nation set apart by God. Circumcision was a physical representation of one’s place in community. Circumcision was a testimony by the parents, family, and larger community, that the child was part of the family and their declaration that they were still followers of God.
It is the larger picture of community, and the vital role that circumcision played in it, that changes the unclean durations based upon the gender of the child. Community was survival, and still is today.