there was nothing "con-consentual" about Mary becoming pregnant by the Holy Spirit.
God chose Mary for this job because He knew that she would say "yes". He would not have chosen someone who would have refused, been too sacred to say yes, or have done a poor job of mothering the Saviour. He would not have chosen someone who was un-Godly, or unable to do what He was asking.
also, you will note that in Luke's Gospel, after the angel explains everything that will happen to Mary, Mary responds by saying "i am the Lord's servant, may it be to me as you have said".
as a side note, there is no Biblical indication of how old Mary was when she was engaged to Yosef. she was clearly a young girl , but as to her exact age, the Bible does not say.