Well, if you don't like that answer, then either Jesus was not fully human, or at best Mary was only a surrogate mother. Does the Church consider Mary only a surrogate? Plus, if Mary was only a surrogate then prophecy is broken since Jesus did not come from the Davidic line. "Son of David" would be a complete lie.
Throwing in genetics is pointless. God doesn't have to follow genetic laws, but it also takes a leap of faith to believe all this really happened. Rather it is obvious that these stories simply reflect the common beliefs of the time.
Then what do YOU think really happened?
The economic "role" of the Holy Spirit in Incarnation was to cleanse Mary's flesh and make her a worthy vessel, the way the Holy Spirit cleanses the bread and wine and makes it the pristine Body and Blood of Christ.
What God "touches" becomes pristine and perfect. So, the material doesn't matter. Adam was made of clay and is fully human even though he didn't have a mother! Why would Jesus be any less human if he took Mary's human nature (the meaning of Mary's "flesh" in this case)?
Plus, if Mary was only a surrogate then prophecy is broken since Jesus did not come from the Davidic line. "Son of David" would be a complete lie.
Jesus' Davidic line is as wishy-washy as it gets. His genealogies don't match, and Jesus denies it Himself when He says "Who is My mother and who are My brothers?..."For whoever does the will of My Father who is in heaven, he is My brother and sister and mother." [Mat 12:48, 50]
In the ancient Middle East, there are no female genealogies. Succession was through the male line only.
Also note another "works-based" reference to those who are Jesus' brothers and sisters.