25 For it became Him who is most pure, and a teacher of purity, to have come forth from a pure bride-chamber. For if he who well fulfils the office of a priest of Jesus abstains from a wife, how should Jesus Himself be born of man and woman? For thou, says He in the Psalms, art He that took Me out of the womb. Mark that carefully, He that took Me out of the womb, signifying that He was begotten without man, being taken from a virgin's womb and flesh. For the manner is different with those who are begotten according to the course of marriage.
Source: Catechetical Lectures (New Advent)