It matters because it relates to the Virgin Birth and to the divinity of Mary's child. Are the nativity stories an essential part of Christianity?
Mary having sex with her HUSBAND relates in NO WAY to the divinity of Jesus Christ, the Son of God. It seems for you; it does. Why is that? Why is it necessary that she remained a Virgin?
It matters because it relates to the Virgin Birth... Technically, no it doesnt. It stops mattering at the manger.
...and to the divinity of Mary's child.
Can you elaborate on that? Are you saying that Jesus was made divine?
Are the nativity stories an essential part of Christianity?
Absolutely. Explain how and why the Virgin Birth (a single event in time-and-space) demands a Perpetual (never ending) maintenance of Mary's virginity.