A man named Bob left this comment on my previous post:
"The virginity of mary is based upon the Septuagint's translation of Isaiah. However, the catholic church doesn't use the Septuagint as its main bible. strange, isn't it? In the Hebrew version of Isaiah the word translates as "young woman," not virgin.'"
Thank you, Bob, for bringing up this interesting point of discussion. For a full discussion of the biblical issues, here is a link to an excellent article by Mark Shea on this topic.
In the Church's history, many writers did in fact point to this text of Is 7:14 in discussing Mary's virginity (in this context it is her virginity before the birth of Christ.) But Catholic teaching on her virginity is primarily based on the texts of the Gospel, in particular Matthew and Luke. For example, in the annunciation Luke writes that the angel Gabriel said to Mary, "The child who will be born of you will be holy, the Son of God." It's very clear from that text that the child is to be miraculously conceived "of the Holy Spirit."
I might add too that not only Catholics but the many Protestants, especially evangelicals, strongly uphold the virginity of Mary before Christ's birth based on the Bible. They would part company with us concerning her virginity later on, because they believe she had other children. So the text from Isaiah can be understood in light of Mary's virginity but it is not the only reason that Catholics and other Christians believe that Mary was a virgin in giving birth to Christ.