Perpetual Virginity

For those who question the Church's teaching of the perpetual virginity of Mary, the Mother of Jesus, the Mother of God. There are some things that have run through my brain while meditating on the mysteries of the Gospel.

Those familiar with the story of Our Lord and Savior Jesus Christ may remember the scene: Gabriel appears to Mary who is betrothed (engaged) to Joseph the carpenter. He hails her as full of grace and proceeds to tell her of the One she is to give birth to.

This is very important.
She then asks him, how can this to be, since I do not know man? (Luke 1:34)
[There are many different translations of this passage, but the point is the same]

Let's look at the context of this question. She is to be engaged to Joseph. Let's look at the fact that she even asks the question. Why would she ask "How is to be? How will this happen?" She's a woman, she's engaged, she's not ignorant. If she had meant to have children, she would not have asked this question.

If she had meant to have children, the Annunciation would have been a confirmation, and the news of Jesus would have been like a "so that's what God wants me to name the child I may have once I'm married". But she asks how! Implying that she had meant to stay a virgin from the beginning.

What're your thoughts on this?