But if anyone feels a doubt as to why the Virgin conceived after she was betrothed rather than when she had no one betrothed to her, or, to use the Scripture phrase, no husband, let me explain that there were three reasons. First, that by the genealogy of Joseph, whose kinswoman Mary was, Mary’s origin might also be shown. Secondly, that she might not in accordance with the law of Moses be stoned as an adulteress. Thirdly, that in her flight to Egypt she might have some solace, though it was that of a guardian rather than a husband. For who at that time would have believed the Virgin’s word that she had conceived of the Holy Ghost, and that the angel Gabriel had come and announced the purpose of God? and would not all have given their opinion against her as an adulteress, like Susanna? for at the present day, now that the whole world has embraced the faith, the Jews argue that when Isaiah says, “Behold, a virgin shall conceive and bear a son,” the Hebrew word denotes a young woman, not a virgin, that is to say, the word is Almah, not Bethulah, a position which, farther on, we shall dispute more in detail. 

As to Jerome’s reasons, they fail for several reasons.  First, Mary’s genealogy can (and was) shown by reference to her father Heli (Luke 3:23), as distinct from Joseph’s father, Jacob (Matthew 1:16).  Second, a single woman who conceived outside of marriage was not guilty of adultery under the law of Moses (Deuteronomy 22:28).  Third, while it may indeed be true that Joseph cared for Mary in Egypt, it is not true that Joseph was merely a guardian and not a husband to Mary.  Moreover, if God merely needed a guardian for Mary, God could easily provide that without providing a husband.   For example, God could have supernaturally strengthened a woman for that task, or God could have sent an angel, or even a lion.