When the angel told Mary she was to become a mother she questioned it. "How can this be?" (Luke 1, 34). An ordinary Jewish girl would have wished and even expected to have a child, particularly if she had already committed herself to marriage through betrothal, as Mary had. St. Joseph was her betrothed husband.
Yet she asked the question, "How can this be?" The Church has always believed and taught that, in agreement with Joseph, Mary had pledged her virginity to God, so that she might be able to dedicate herself more fully to the divine Will.
The Church teaches also that Mary always remained a virgin. She had no child but Jesus. There are some references in the Gospels to "the brothers of the Lord", but brothers among the Jews did not necessarily mean children of the same parents. It included cousins and relatives. Thus in the Old Testament Lot is called the brother of his uncle Abraham, and Jacob is called the brother of his uncle Laban.