It's not just a Catholic thing.
In today's world, where in vitro fertilization is now common, I'm amazed that so many still find the truth of the virgin birth so hard to believe. Is it really that much of a stretch, knowing what we now know?
Actually, assuming they had turkey basters or the equivalent in ancient times, it shouldn't be that much of a stretch for anybody.
Although I guess it depends on your definition of "virgin."