Americans tend to be very prudish collectively. Perhaps the historical roots of Puritanism remains relevant to this day. Perhaps it is something that many Americans still have to contend with.
One thing for sure: this awkward reserve that Americans tend to have on the subject of sex is most definitely not shared by the entire rest of the world!
Many Europeans, in particular, feel that the social mores that Americans subscribe to add a stigma of shame regarding the human body, and that this is generally unhealthy. Nudity traditionally has been strictly forbidden, although this has been easing a bit in recent years.
Still, Americans tend to be far behind their European counterparts in this area.
Here was an interesting link on the subject, where we get a glimpse about what Europeans think of American sex norms:
Here are 5 American sex norms Europeans think are crazy Anna Pulley, AlterNet ANNA PULLEY, ALTERNET 11 JUL 2015