At the time of the Annunciation, Mary was betrothed to Joseph. Jewish marriages took place in two stages: betrothal and wedding. Mary and Joseph’s betrothal had been completed, but the second part of the ceremony (which ordinarily took place at a later date than the betrothal) had not yet occurred. While Mary was not an “unwed mother” in the sense that we understand “unwed mothers” today, her marriage to Joseph had not yet been completed.
Some people become offended at the idea of calling Mary an “unwed mother” because they believe such a description implies sinfulness or shameful circumstances and so should not be imputed to Mary, who is sinless. For the purposes of theology, we should be careful to make the proper distinctions as to Mary’s status; but, for the purpose of pastoral care, if it helps an unwed mother to identify with Mary and to find courage from Mary’s courage, I don’t think it dishonors the Blessed Mother to offer her experience as a model for an unwed mother.