The stereotype that Catholic girls are more promiscuous than Protestant girls is explained not by actual sexual activity--an untrustworthy measuring yard, as there has not been, to my knowledge, an actual scientific study comparing the two groups--but as an exercise in what Edward Said calls "the Other" in his book Orientalism.
In American society, which is predominantly Protestant, the Catholic has long been seen as "the Other"--a designation of some group which is not part of the majority, and thus is used as a projection of the fears and desires of said majority. Let us not forget that it was widely believed up until the election of Kennedy that Catholics were in the United States to overtake it and hand it over to the pope. Catholics--particularly through their main (most populous) ethnic groups of Irish, Italian, and Latino/Hispanic--have long been seen as corrupt drunks, tied to all sorts of crime and political machinations. While this may have some basis in historical fact--for instance, Tammany Hall, the Mafia--it is still a stereotype.
Much in the same way should we view the stereotype of the sexually promiscuous and precocious Catholic girl. Obviously, while there are certainly some Catholic girls who do engage in sex, there are also some who do not. If one were to do an actual study, the numbers would likely be comparable to those of non-Catholics. However, in our quasi-puritan society--where sex is both reviled and celebrated*--the idea of young girls having sex is not placed upon the image of the common Protestant girl, but on the semi-exotic species of Catholic girl, who belongs to a religion misunderstood and viewed with suspicion.** Catholics have in the past attended "parochial" schools and lived in ethnically-centered urban neighborhoods, which lead to less interaction with the Protestant majority. This unfamiliarity, combined with such things as the "catholic school girl uniform"--usually a pleated skirt and oxford shirt--have lead the Catholic girl to be held as an object of exoticism. Usually white, she is more familiar and thus a safer object of sexual desire in the Protestant world, but she is still an outsider, and thus not quite acceptable. Moreover, it is that outsider status which makes it possible to reduce her to a sexual stereotype, much like the "bootylicious" African-American female, the "quiet" Asian geisha-in-waiting, the "sexy" Latina. It is not that these groups have more sex--it is that in American society, they are expected, as outsiders, to have more sex, which is denied in Protestant culture, but practiced just as much.
Finally, I cannot leave this subject without touching upon the birth control issue. I'm sure many of you are familiar with the film Monty Python and the Meaning of Life, with its famous "Every Sperm is Sacred" musical number. This film quite accurately lampoons the stereotypes attatched to Catholic and Protestant sexual practices in Anglo-American culture--that Catholics have a lot of sex but don't practice birth control, and thus produce more children and remain in poverty, while Protestants are sexually repressed, infrequently have sex, and thus have no actual need for birth control. Studies have shown that many Catholics in America do in fact practice birth control and safe sex, disregarding what they feel is an out-dated teaching of their religion. Some do this willingly, others are filled with guilt.
*Not unlike the original Puritans, for whom sex was certainly normal--they were not Shakers--but for whom there was still the idea that sexuality could easily turn sinful, especially in a woman who enjoys sex. This notion was quite at odds with the scientific beliefs of the time, which was that a woman must have an orgasm in order to conceive, and thus it was the man's job to pleasure her. However, it is a concept more in line with Aristotelian thought, namely that movement and pleasure for a woman would not result in conception.
**I am no apologist for the Catholic Church, especially after the recent sex scandals. However, there are many misconceptions about the Church and its adherents, and a certain lingering suspicion that Catholic practices are strange and unnatural, or at least any stranger than any other religion.