From Martin Luther:

In our Christian Creed we confess that Christ was conceived and made man or was incarnate (if I may so speak), that He became a real human being by assuming a body. We confess that He assumed genuine flesh and blood from the Virgin Mary, that He did not pass through her as the sun shines through a glass but brought her virgin flesh and blood with Him. If this had taken place only with the cooperation of Mary, the Babe would not have been pure. But though Mary has been conceived in sin, the Holy Spirit takes her flesh and blood and purifies them; and thence He creates the body of the Son of God. This is why it is said that “He was conceived by the Holy Ghost.” Thus He assumed a genuine body from His mother Mary, but this body was cleansed from sin by the Holy Spirit. If this were not the case, we would not be saved.

From J. Gresham Machen:

We do not mean, in the first place, merely that the virgin birth was important for God’s plan; for that goes without saying if the virgin birth was a fact. If Jesus Christ was really born without a human father, if that really was God’s way for our Saviour to enter into the world, then it may certainly be assumed that it was the best way and that any other way would have been wrong. We are not concerned now to assert anything so self-evident as that. But what we do assert now is not only that the virgin birth was important as an event, but that it is important for us to know — that we could not have remained ignorant of it without loss.”