While the write-up above is in some sense true--that earlier cultures such as the Sumerians, or even up through the Roman Empire--were more open about sexuality than modern American culture, what one must not forget is that this "open sexuality" was religiously based. It was believed that the gods' sexuality--and human sexuality--was linked to the fertility of the land. In the example of Inanna and Dumuzi (or Aphrodite and Adonis, or Cybele and Attis), we are not only discussing sex, but vegetation. The fertile nature of the gods was of utmost importance.
This was also related, then, to human sexuality. Many ancient religions had ritual sex, orgies, and ritual marriages between the land and the king. In the example of orgies, this was not only because of simple human libido, but was driven by a hope to sexually arouse the gods, which would then bring forth fertility in the land. Wine was also a staple of the orgy, as it was seen as the blood of the gods--and proof of the earth's fertility. What we forget is that food--grains, particularly--were also important at these celebrations. Bread was the body of the god. This is all highly reminiscent of the Catholic rite of the Eucharist, which is only logical, as Judaism was long rooted in and struggled with earlier fertlity cults, even worshiping the gods named above. {This statement was disputed--but the Eucharist--not the Mass--is based on the gospels, which were written by Jewish Christians ca. 70--100 CE. They were influenced by Judaism as well as pagan cults.)
The marriage of the king to the land was also a widespread phenomenon, from ancient Babylon to medieval Ireland. One can see a connection, too, between the castrating gods Attis and Adonis, and that of the Celtic Fisher King--all of whom the fertility of the land rested upon.
My point here is that sexuality was not only sexuality in these cultures, but was symbolic of life, of fertility. In the modern world, most of us know there are no gods who will be sexually aroused, who demand sacrifices, who need us to worship them. We don't need orgies or sacred marriages in order to be sure that crops will grow. Sex is a recreational activity--it isn't even necessary for the creation of another human (see in vitro fertilization or cloning for examples of how sexual intercourse has become essentially useless for propogation--fun but unnecessary). Sex is no longer sacred, just as food is no longer sacred--we can easily go to a supermarket and pick up something to eat, and no grace must be said, no sacrifices must be made. There is no punishment if we do not take part in orgies or sacrifice a bull. Life will go on without the gods.
That said, while American discomfort with sexuality can be traced to an embracing of Victorian morals (NOT the Puritans, who saw sex as an act demanded by God--"be fruitful and multiply"), and a neurotic attempt to control human desire and the body--also illustrated by America's fascination with diets. It is not because we are somehow more repressed than the Sumerians where sexuality is concerned. It is actually due to the strange dicotomy of American culture. Supposedly a classless society, there are definite classes: white collar and blue collar, middle class and working class, etc. (I will, however, add that these classes can be transgressed much more easily than in other societies.) Sex is taboo, yet pornography is the biggest moneymaker in the entertainment industry. We should all be thin and diet, yet we're a grossly overweight nation.
The discomfort with sexuality is not the disease, but symptomatic of a larger issue at hand--strict control being emphasized over a population that constantly asserts its "freedom." If society were to actually practice freedom, chaos would result; the appearance of freedom, with the institution of societal limits, social punishment for transgressions, is used to keep the larger social order, as the semi-classless society is one which can occasionally be transgressed ("movin' on up"). The image of freedom runs counter to the reality--or, as William S. Burroughs puts it, "A functioning police state needs no police." Instead, we have pressure valves: we're encouraged to be thin, but encouraged to eat Big Macs. We're encouraged to be celibate, but encouraged to be like the women in Sex and the City. Yet with each minor release, each opening of the valve, there is the implication of guilt, thus reinforcing the social standard.