As I said in my other post (that JSP conveniently ignored), the "morning after pill" has essentially the same effect as standard birth control pills, it's just a stronger dose. BCP's, taken normally do the same three things that the "morning after pill" can do. It's just that, based on the timing, taking it after may be too late for the first two defenses (preventing ovulation and hindering the advance of sperm).
But yeah, according to the Vatican, sex is for procreational purposes only...