Why were women unclean during their period in the Old Testament? Also, why were the number of unclean days different for the birth of a male child vs. a female child? Why doesn't this apply today?
Why were women unclean during their period in the Old Testament?
We need to remember that being in a state of "uncleanness" was not the same as sin. It's more like being put on the bench during a game. I believe the Old Testament's emphasis on cleanness and uncleanness was to weave the importance of holiness and "separation unto the Lord" into the everyday understanding of what it meant to serve the true and living God. The distinction between cleanness and uncleanness functioned as a continual reminder of the difference between God (holy) and God's people (sinful and fallen).
Actually, I believe the ritual uncleanness of a woman's menstrual period had two purposes. First, it kept the messiness more contained by restraining her activities, especially sexually. Secondly, when sexual relations were forbidden for seven days each month, it was a built-in anticipation builder for both husband and wife for when they could come back together again. Many married couples know the joy of "reunion sex." God's "off-limits for seven days" rule insured "reunion sex" without somebody having to go away! <smile>
Also, why were the number of unclean days different for the birth of a male child vs. a female child?
I couldn't find a single commentator who could come up with a reason apart from God's right to make the rules. However, since the New Testament teaching is equal value of the sexes (Gal. 3:28, "In Christ there is no male or female"), it may be that the purpose of the gender INequity in the Old Testament was to set up the contrast for the glory of grace in the New Testament.
Why doesn't this apply today?
It doesn't apply today because the purpose of the Old Testament civil law has been fulfilled. The laws were designed to protect and provide for the purity of the Jews until the Messiah came. Now, Christ has torn down the barrier between Jew and Gentile, and the Old Testament law was a huge part of that barrier--which is no longer necessary. (It should be noted that moral laws, such as what we find in the Ten Commandments, will never pass away because they are rooted in the very character of God.)
Hope this helps!