We credit our prudishness on the long ago extinct race of beings known as Puritans. A race of beings that once populated New England, famous for burning witches at the stake, and wearing designer black & white outfits before it was fashionable...at a time long before we fired our muskets at red coats.
I challenge anyone here in the US of A to find one Puritan. I've not seen one myself - they are as physically prolific as the Carolina parakeet....but - they some how they live on. :-)