How Jesus instructed us that Mary remained a virgin until the end

Indeed, when he was lifted up on the Cross, Jesus commended John to the mother, saying, "Woman, behold your son." And to the disciple, "Behold your mother." And from that moment on, "the disciple took her into his home" (cf. Jn 19:26-27).

With these words, the Savior affirms that Mary did not bear another son after him.

For, if she had had other children, the Savior would not have ignored them and entrusted His Mother to someone else; nor would she have become someone else’s mother. She would not have [abandoned her own] to live with others, knowing well that it ill becomes [a woman] to abandon her husband or her children. But, since she was a virgin and was his mother, he gave her as a mother to his disciple, even though she was not really John’s mother, because of his great purity of heart and because of her untouched virginity.”

Saint Athanasius, De Virginitate, in Le Museon 42: 243-44