Thomas Aquinas

Without any hesitation we must abhor the error of Helvidius, who dared to assert that Christ's Mother, after His Birth, was carnally known by Joseph, and bore other children. For, in the first place, this is derogatory to Christ's perfection: for as He is in His Godhead the Only-Begotten of the Father, being thus His Son in every respect perfect, so it was becoming that He should be the Only-begotten son of His Mother, as being her perfect offspring.

Secondly, this error is an insult to the Holy Ghost, whose "shrine" was the virginal womb ["Sacrarium Spiritus Sancti" (Office of B. M. V., Ant. ad Benedictus, T. P.), wherein He had formed the flesh of Christ: wherefore it was unbecoming that it should be desecrated by intercourse with man.

Thirdly, this is derogatory to the dignity and holiness of God's Mother: for thus she would seem to be most ungrateful, were she not content with such a Son; and were she, of her own accord, by carnal intercourse to forfeit that virginity which had been miraculously preserved in her.

Fourthly, it would be tantamount to an imputation of extreme presumption in Joseph, to assume that he attempted to violate her whom by the angel's revelation he knew to have conceived by the Holy Ghost.

We must therefore simply assert that the Mother of God, as she was a virgin in conceiving Him and a virgin in giving Him birth, did she remain a virgin ever afterwards...

Some, as Jerome says on Matthew 12:49-50, "suppose that the brethren of the Lord were Joseph's sons by another wife. But we understand the brethren of the Lord to be not sons of Joseph, but cousins of the Saviour, the sons of Mary, His Mother's sister." For "Scripture speaks of brethren in four senses; namely, those who are united by being of the same parents, of the same nation, of the same family, by common affection." Wherefore the brethren of the Lord are so called, not by birth, as being born of the same mother; but by relationship, as being blood-relations of His. But Joseph, as Jerome says (Contra Helvid. ix), is rather to be believed to have remained a virgin, "since he is not said to have had another wife," and "a holy man does not live otherwise than chastely."