The Lex Scantinia (less often Scatinia) is a poorly documented Roman law that penalized stuprum (criminalized sexual behavior or "sex crime") against a freeborn male minor (ingenuus or praetextatus). The law may also have been used to prosecute adult male citizens who willingly took a passive role in having sex with other men. It was thus aimed at protecting the citizen's body from sexual abuse but did not prohibit homosexual behavior as such, as long as the passive partner was not a citizen in good standing. The primary use of the Lex Scantinia seems to have been harassing political opponents whose lifestyles opened them to criticism as being passive homosexuals or pederasts in the Hellenistic manner.

The law may have made stuprum against a minor a capital crime, but this is unclear: a large fine may have been imposed instead, as executions of Roman citizens were rarely imposed by a court of law during the Republic. The conflation of the Lex Scantinia with later or other restrictions on sexual behaviors has sometimes led to erroneous assertions that the Romans had strict laws and penalties against homosexuality in general.

Latin has no words that are straightforwardly equivalent to "homosexual" and "heterosexual." The main dichotomy within Roman sexuality was active/dominant/masculine and passive/submissive/"feminized." The adult male citizen was defined by his libertas, "liberty," and allowing his body to be used for pleasure by others was considered servile or submissive and a threat to his integrity. A Roman's masculinity was not compromised by his having sex with males of lower status, such as male prostitutes or slaves, as long as he took the active, penetrating role. Same-sex relations among Roman men thus differed from the Greek ideal of homosexuality among freeborn men of equal social status, but usually with some difference in age (see "Homosexuality in ancient Greece" and "Pederasty in ancient Greece"). The adult Roman male who enjoyed receiving anal sex or performing oral sex was thought to lack virtus, the quality that distinguished a man (vir).

The protective amulet (bulla) worn by freeborn Roman boys was a visible sign that they were sexually off-limits. Puberty was considered a dangerous transitional stage in the formation of masculine identity. When a boy came of age, he removed his bulla, dedicated it to the household gods, and became sexually active under the patronage of Liber, the god of both political and sexual liberty. Pederasty among the Romans involved an adult male citizen and a youth who was typically a slave between the ages of 12 and 20.

As John Boswell has noted, "if there was a law against homosexual relations, no one in Cicero's day knew anything about it." Although the Lex Scantinia is mentioned in several ancient sources, its provisions are unclear. It penalized the debauchery (stuprum) of a youth, but may also have permitted the prosecution of citizens who chose to take the pathic ("passive" or "submissive") role in homosexual relations. Suetonius mentions the law in the context of punishments for those who are "unchaste," which for male citizens often implies pathic behavior; Ausonius has an epigram in which a semivir, "half-man," fears the Lex Scantinia.

It has sometimes been argued that the Lex Scantinia was mainly concerned with the rape of freeborn youth, but the narrowness of this interpretation has been doubted. The law may have codified traditional sanctions against stuprum involving men, as a forerunner to the Lex Julia de adulteriis coercendis that criminalized adultery involving women. The early Christian poet Prudentius makes a scathing joke that if Jupiter had been subject to Roman law, he could have been convicted under both the Julian and the Scantinian laws.

Only youths from freeborn families in good standing were protected under the law; children born or sold into slavery, or those who fell into slavery through military conquest, were subject to prostitution or sexual use by their masters. Male prostitutes and entertainers, even if technically "free," were considered infames, of no social standing, and were also excluded from the protections afforded the citizen's body. Although male slaves were sometimes granted freedom in recognition of a favored sexual relationship with their master, in some cases of genuine affection they may have remained legally slaves, since under the Lex Scantinia the couple could have been prosecuted if both were free citizens.

The infrequency with which the Lex Scantinia is invoked in the literary sources suggests that prosecutions during the Republican era were aimed at harassing political opponents, while those during the reign of Domitian occurred in a general climate of political and moral crisis.