Some excellent scriptural exegesis regarding Mary's Perpetual Virginity can be found at the University Dayton's Website
http://campus.udayton.edu/mary/questions/yq/yq195.html
An excerpt from the article reads "But we do know dogmatically as Catholics that Mary would not have "bled" during her giving birth to Jesus. We know this from the dogma of Mary's Perpetual Virginity - before, after, and particularly during the birth of Jesus. It is an article of divine Catholic faith that Mary preserved her virginity - inviolate from any physical damage or destruction."
There are plenty of women are are not "intact" but are yet virginal. IAC, the details of her birth seem unimportant when one confronts the real miracle of the Virgin giving birth to a child without a human father. THAT is what the rationalist cannot accept.