Imagine you’re standing on the beach, when suddenly a UFO appears, and out of a beam of light descends a being not like any you’ve seen before. The alien, speaking some intelligible tongue, seeming to concentrate for a moment, then turns to you and opens its mouth.
“Describe to me, this… ‘vagina.'”
You fumble for the right words, but realizing that as an alien he may have no understanding of Earth culture, you look for something as a point of reference. Finding a bed of mussels, you grab one and continue your description.
A number of people have written about Tsukihime and Fate/Stay Night writer Nasu and his penchant for describing female genitalia using mollusk-themed vocabulary. When you think about it though, it’s a good physical description for someone like an extraterrestrial who has never seen human female reproductive anatomy. And with the nature of censorship in Japanese pornography, it’s possible (though perhaps unlikely, given the power of the internet in this age) that a virgin in Japan has never seen a real vagina, in photos or otherwise.
So in conclusion, Type Moon is simply preparing for the future when we will make contact with our brothers across the stars.