These are short questions that probably can't ever be fully answered and require books to be answered even semi-adequately. That said, a good answer probably has some element of the U.S.'s strange relationships with sexuality and Christianity, going back to the particular kinds of European settlers who showed up here. This is often referred to as "Puritanism," although the Puritans actually had a much more complicated and less prim experience than is commonly supposed with the slur "Puritanism."

At the same time, in the last several decades (maybe 50 or 60), there have been various strands of feminism; two particularly noticeable parts could be labeled as "sex positive" (Camille Paglia is a good person to read on the subject) and... I actually can't think of a good label for the other one. But the other one actually has quite a bit in common with the old-school and religiously motivated views of sexuality as dangerous and in need of extreme restraint or channeling into "appropriate" spaces. This one has quite a bit of currency, currently, and it has a lot of continuity with past aspects of U.S. culture.