Mary was simply asking how God was going to bring about this miracle. The angel told her how it was going to be done. You will find this part is not covered in Isaiah 7. Mary was simply curious This is a reasonable explanation, but it still does not explain why a girl about to be married presumes the predicted conception to be in any other manner but in the manner how every other girl conceives a child upon marriage.
Also, this is not the only reason to believe in her perpetual virginity, it is simply the most suitable for Protestant style prooftexting. There are also (off the top of my head)
- The manner in which everyone calls her The Virgin, starting in Luke 1. Why is it noteworthy that a young girl about to be married is a virgin?
- "And the Lord said to me: This gate shall be shut, it shall not be opened, and no man shall pass through it: because the Lord the God of Israel hath entered in by it, and it shall be shut." (Ezechiel 44:2)
- As a spouse of the Holy Ghost, Mary would not have intercourse but with the Holy Ghost
- Endless amount of patristic literature calling her ever virgin.