There's no positive statement in Scripture that says that Mary was a perpetual virgin. Then again, there's no positive statement in Scripture saying that she wasn't and that was the point of this post.
Scripture, though normative, is not the sole source of faith, as far as the Catholic Church is concerned. So the absence of any positive statement in favor of Mary's Perpetual Virginity doesn't bother us. The testimony comes from the Deposit of Faith handed down in Tradition, to which Scripture belongs and from which Scripture is rightly interpreted.
-Theo
In other words, you just make stuff up.