9 And that you may learn more exactly that in the Divine Scriptures it is not by any means the natural father only that is called father, hear what Paul says:— For though you should have ten thousand tutors in Christ, yet have ye not many fathers: for in Christ Jesus I begot you through the Gospel. For Paul was father of the Corinthians, not by having begotten them after the flesh, but by having taught and begotten them again after the Spirit. Hear Job also saying, I was a father of the needy: for he called himself a father, not as having begotten them all, but as caring for them.
And God's Only-begotten Son Himself, when nailed in His flesh to the tree at the time of crucifixion, on seeing Mary, His own Mother according to the flesh, and John, the most beloved of His disciples, said to him, Behold! Your mother, and to her, Behold! Your Son: teaching her the parental affection due to him, and indirectly explaining that which is said in Luke, and His father and His mother marvelled at Him: words which the tribe of heretics snatch up, saying that He was begotten of a man and a woman.
For like as Mary was called the mother of John, because of her parental affection, not from having given him birth, so Joseph also was called the father of Christ, not from having begotten Him (for he knew her not, as the Gospel says, until she had brought forth her first-born Son), but because of the care bestowed on His nurture.
Source: Catechetical Lectures (New Advent)