What about American squeamishness? Why are Americans so freaked out by sexuality and nudity in mainstream channels? I feel like this mentality is dying out hard, though.
Because old social rules about how civilization should be a thing. Technically every single social rule we exert on ourselves is entirely arbitrary. Some were derived from common sense. For example: Wear clothes so you won't fucking freeze to death during the winter, or having your more sensitive parts get constantly hit and torn up by plant life. It's ways that we orchestrate tribal behaviour and pass along important lessons from parent to child. "Wear clothes." "But why?!" "Because that's just how society works."
One of those ways we orchestrated sex rules goes all the way back to the era of the Roman Empire and the Catholic Church. They wanted a way to increase church influence in the lives of the everyday commoner, so they created stable family lives via monogamous marriage. However, how do you enforce marriage as being sacrosanct? Punish those who stray from it. This sounds nefarious, but for the times, it made sense: You wanted stable family units in stable villages who could create lots of food and other stuff you needed instead of killing each other over petty disputes about who banged the most waifus. They further cemented it by the idea of making your body sacred and your sex parts--and your compulsion to have sex--tied to sin. It required specific kinds of blessings and the like in order for it to go from a sinful compulsion to a holy act of love--marriage. Who controlled marriage? The church did.
Now carry this mentality over several hundred years, and it becomes ingrained as a societal norm. "Why do we get married?" "Because that's just how things work." "Why is love monogamous?" "Because that's just how things work." Now, let's take a glance at early colonial history in the Americas. What was a big thing?
The four great awakenings. America and religion are inexorably tied at the hip not in law, but in culture--along with most of western civilization. Some of these carry-overs included ideas about how and why sex should be a thing. It should be no great coincidence to you that some of the most ardent and strict fighters against positive sexual imagery or sexual imagery in general--such as pornography, or adult art--are some of the most conservative Christians you can meet. If you think you're awkward about sex, try the Catholic Church, that still takes a negative view about the usage of condoms, and believes in abstinence only education.
Hmm... Abstinence only education...
Where have I heard this before...
tl;dr: Arguments of morals aside (because you can legitimately argue it either way), you can trace the sex-negative ideas of western culture in general back to the late Roman Empire and the Catholic Church. They made for excellent control mechanisms and ways of breaking clans into smaller family units, that went from being dependent on a tribe to being dependent on a more unifying factor--religion. Fast forward to the heavily religious charged America and it shouldn't be much of a surprise that the prevailing mentality of millions of people is still very similar to that of the root culture they grew up in--part of which is religious in nature (Christian specifically), which has always had a traditionally negative view of sex and the naked human body.
