The Orthodox tradition regarding the early life of Mary explains all of this. None of it is in the Bible explicitly, but it is compatible with the Biblical accounts.
Monkfan makes a valid point. If an angel appeared to a young woman planning to marry, and told her that she would conceive and bear a son, why would she assume that it meant "conceive -- this very instant?" It would have been most reasonable for her to assume that the angel meant that this would happen after she was married and began to "know" her husband.
The Biblican account is rather that she said "how can this thing be?" This makes sense if you understand that she had vowed to remain a virgin.
According to our tradition, she lived at a time when there was no "single option" for women. One married. Period.
Our tradition is that at that time, female who were pre- and post-childbearing capability lived in the temple complex, doing temple work -- cleaning, sewing, etc... She was dedicated to God and was brought to the temple at the age of 3 to begin this life of prayer and service to God.
When she, to put it delicately, was approaching the age when her presence would make the temple "unclean," she had to leave. A life of virginity was unknown at that time for women, and the only choice was to marry. Her family knew of her desire to remain a virgin, and the decision was made to betroth her to an elderly uncle who was a widower. She would be under his protection and be cared for by him, but would be able to continue to live as a virgin, dedicated to prayer.
After his death, she would be able to live as a widow, and eventually could return to the temple when she would no longer make the temple "unclean" -- much as the elderly Prophetess Anna lived in the temple as a widow, and as did other elderly widows of great piety.
It was at this point that the Archangel Gabriel appeared to her -- in light of this, it is understandable why she would ask "how can this thing be?" Had she been betrothed to a virile young thing with whom she planned to get active and have children, why would she have asked such a question?
Sorry, everything in the Bible that is written about Mary and Joseph points to the fact that they were a married couple in every sense of the word.
:-)
There is no reason to expect or believe that Mary would of been approached by an angel so that the angel could announce her impending motherhood had this been just a normal run of the mill pregnancy.
And also, that totally destroys your argument that Mary was sworn to be and remain a virgin.
Better still, let us look to what the Bible says and why Mary was in amazement.
Mary knew the sign of the messiah.......
Isaiah 7:14 Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.
What you and Monkfan are saying is not consistent with Luke. When the angel of the Lord approached Mary and told her she would conceive and bear a son, she asked the most appropriate question,
According to our tradition, she lived at a time when there was no "single option" for women. One married. Period.
I would also suggest that you are forgetting Joseph in this whole argument. For Joseph to have married someone who was already impregnated would have been a terrible thing to do for a good Jewish man.
Mary's submission to God's calling is what's make this such a beautiful event for she was willing to go through not only the social stigma but possibly being stoned to death according to custom. Joseph's submission is what is often overlooked because he believed God. Though he was engaged and should have properly given Mary a certificate of divorce, was obedient to the Lord and carried through on his commitment.