For "he did not know her" - it says - "until she gave birth to a Son, her firstborn." But this could make one suppose that Mary, after having offered in all her purity her own service in giving birth to the Lord, by virtue of the Holy Spirit, did not subsequently refrain from normal conjugal relations. That would not have affected the teaching of our religion at all, because Mary's virginity was necessary until the service of the Incarnation, and what happened afterward need not be investigated in order to affect the doctrine of the mystery. But since the lovers of Christ [that is, the faithful] do not allow themselves to hear that the Mother of God ceased at a given moment to be a virgin, we consider their testimony sufficient (St. Basil the Great, Homily [PG 31, 1468]).
Luther, Marginal Note at Matthew 1:25 in Luther's Bible (1545): “It should not be understood that Joseph knew Mary afterwards. Rather this is a manner of speaking in the Scriptures. As in Genesis 8, the raven did not come back “until the soil dried out;” the Scripture does not mean that the raven came back after that. So also it does not mean to say that Joseph knew Mary afterwards.”
"Christ, ..was the only Son of Mary, and the Virgin Mary bore no
children besides Him... "brothers" really means "cousins" here, for
Holy Writ and the Jews always call cousins brothers." -Martin Luther
(Sermons on John, chapters 1-4.1537-39).
"He, Christ, our Savior, was the real and natural fruit of Mary's
virginal womb.. .This was without the cooperation of a man, and she
remained a virgin after that." (Ibid.)
"God says... "Mary's Son is My only Son." Thus Mary is the Mother
of God.". (Ibid.)
"God did not derive his divinity from Mary; but it does not follow that
it is therefore wrong to say that God was born of Mary, that God is
Mary's Son, and that Mary is God's mother...She is the true mother of
God and bearer of God...Mary suckled God, rocked God to sleep, prepared
broth and soup for God, etc. For God and man are one person, one Christ,
one Son, one Jesus. not two Christs. . .just as your son is not two
sons...even though he has two natures, body and soul, the body from
you, the soul from God alone." -Martin Luther (On the Councils and the
Church, 1539)