Why is the virgin birth necessary for Jesus to be sinless?
The virgin birth is necessary because the blood system of a fetus comes only from the father, not the mother. Had Mary's blood entered Jesus, He could not have been sinless. With no human father, Jesus had divine, untainted blood and was incorruptible.
Les explains that the physiological fact that the mother's blood never enters the fetus is the basis of the doctrine of the virgin birth. The mother's blood has nothing to do with the fetus; the whole blood system comes from the father. Therefore, because Jesus did not have a human father, He did not have human blood but divine blood. This meant He could be sinless, His blood was divine and untainted, and He was incorruptible. Had Mary's blood interchanged with the fetus while He was in the womb, that would not have been true. Les says this is basic to our salvation, and he also points to cattle breeding experiments showing the same principle: the blood of the mother never enters the calf.