Maybe I can join in before the thread gets locked.
The origin question was "If abstinence is the only effective form of birth control approved by the Catholic church then how did a virgin get pregnant?".
While this is a dig at the catholic doctrine, there are far more ridiculous beliefs out there. Mormons, I'm looking at you.
But back to the question. Catholics don't necessarily state that abstinence is effective, merely that it is the only acceptable birth control. The fact that a part of the doctrine is that a virgin gave birth suggests that they know it's not a 100% thing. Basically saying as long as you don't hang around with gods, abstinence will probably work.
It doesn't matter whether said virgin was catholic, only that catholics believe that a virgin birth happened. To raise the fact that Mary wasn't catholic does rather imply that her faith was relevant to the whole virgin birth, which it wasn't. This is where the "only applies to catholics" part comes from.
Incidentally, nowhere in any of this is it explicitly stated that the question refers to Mary. It turns out virgin births tend to be all the rage in religion with some other examples being Perseus, Krishna, Horus and in some versions, Buddha.
Any questions?