Women do not carry the "seed" - they carry the "egg". So the term "her seed" in Genesis 3:15 and "the Holy Spirit will come on you, and the power of the Most High will overshadow you." is the "seed" being miraculously placed in her by God. "Since the children have flesh and blood, he too shared in their humanity." (NIV) Forasmuch then as the children are partakers of flesh and blood, he also himself likewise took part of the same; that through death he might destroy him that had the power of death, that is, the devil; (KJV)
What did he share? flesh and blood . . .
Do you think that if the Israelites had prepared a lamb that had spot, wrinkle or blemish for Pentecost would have been acceptable to God and that the angel of death would have passed over them? The lambs were to be without "defect" to be acceptable. . . having a "sinful human nature" would be a "defect" not acceptable as a sacrifice for the sins of the world.