Pope Shenouda III
The sentence “before they came together, she was found with child”
St Luke the Evangelist aims at proving that Christ was
conceived of a virgin who did not know any man, for two reasons:
(a) The fact that Christ was not born through the natural way from two parents as all people are, but was born of a virgin, is proof of His Divinity; proof that He was born of the Holy Spirit as expressed by the angel: "...for that which is conceived in her is of the Holy Spirit" (Matt. 1: 20).(b) His birth from a virgin convinces us that He did not inherit Adam's sin. Thus He would be able to save us since the One without sin is able to die for the sinners.
Therefore the Apostle concentrated on the Virgin not knowing any man before giving birth to Christ in order to prove the Lord's Virgin Birth. But the fact that the Virgin Mary did not know any man after giving birth to Christ is a self-evident truth.