A commenter writes:
The general topic of urban law legends always interested me. I traced the belief that if three of your friends testify to sex with a statutory rape complainant, she is declared a prostitute and the charge is dismissed, back to the common law allowing cross-examination of sex victims' sex lives.
That's not quite a legend, it turns out. Some states did indeed provide the underage person's past sexual history as a defense to at least certain kinds of statutory-rape-related charges, and at least one still does. Texas Penal Code §.; 22.011, for instance, used to provide, until 1993, that it was a defense to statutory rape that "the child was at the time of the offense 14 years of age or older and had prior to the time of the offense engaged promiscuously in conduct described in that subsection." The same rule seems to have existed in Mississippi at least until at least the mid-1980s, and in Florida and Tennessee until the 1990s. Likewise, Mass. Stat. ch. 272 § 4 to this day prohibits "induc[ing] any person under 18 years of age of chaste life to have unlawful sexual intercourse."
Likewise, the Model Penal Code (§ 213.6), proposed in the early 1960s, provided for a promiscuity defense, and cited some past statutes (in South Carolina, Tennesse, and Virginia) as authority, while noting that the then-modern trend had been to reject such a defense. "[P]roof of prior sexual promiscuity," the Code's drafters reasoned, "rebuts the presumption of naivete and inexperience that supports the imposition of criminal liability." Pennsylvania had enacted this provision in 1972, but has since repealed it (as best I can tell, in 1976).
I'm not sure that the underage person's past sexual history is a complete defense today anywhere in the U.S. -- Massachusetts, for instance, generally prohibits sex with under-16-year-olds, without a "chaste life" qualifier. So don't you folks start getting any ideas here. Still, the promiscuity exception to statutory rape did exist in at least some jurisdictions, until not so long ago.