On Mary's Virginity

Forgive me while I state the obvious:

When Archangel Gabriel told Mary that she would conceive and bear a child, this should not have been a big deal. Obviously the part about "Son of the Most High" would have filled her with awe, but not the part about actually getting pregnant. After all, Mary was engaged, so that is pretty much the normal course of things: get married, have sex, make babies. But to Mary, there was something wrong in all of this. She asks the angel, "How can this be since I do not know man?"

If, prior to this encounter, Mary had planned on having marital relations with Joseph, her question would make no sense. The angel had told her she was about to conceive, not that she already had conceived. The only way her response to Gabriel makes sense is if Mary does not plan on having sex with Joseph.

Now, surely most people who have read Luke closely already knew this, but somehow this evaded me for about 5 years after (re-)becoming Catholic. It's not that I didn't accept Mary's post-nuptial virginity, just that I never read this passage in that light - and I surely read it many times on my own, not to mention the several times I would have heard it in the liturgy or encountered it written meditations or lectures.

But maybe it's not so obvious. Many of our Protestant brethren, many of whom accept Scripture as inerrant, castigate us Catholics for denying that Jesus had full blood-siblings. What gives?

Anyway, what brought this to mind is that Edward Sri addressed this very issue at greater length in this essay from a recent issue of Lay Witness Magazine.