It is a very ancient tradition within the Church that our Lady took a vow of perpetual chastity when she was a young girl and that she was brought up in the Temple precints in Jerusalem.

The marriage was in part to avoid scandal (as we can surmise from the Sacred Scriptures.) Our Lord also needed (to the extent that he allowed Himself to experience his humanity, and show Himself to be not only truly God, but also truly man. He required a Human father to provide for Him and to be an example of a holy family life.