"Modern sexuality offers a two-tiered dichotomy based on sexual preference. A homosexual is characterized by his exclusive sexual preference for same-sex relationships. Similarly, a heterosexual favors exclusive sexual relationships with members of the opposite sex. Ancient sexuality, on the other hand, finds its basis in status. The active partner, i.e. the partner of a higher social status, assumes the role of the penetrator; whereas, the passive partner, i.e. the partner of inferior social status, takes on the penetrated position.(www.princeton.edu/~clee/paper.html) - Malakos
Our modern preoccupation with sexuality has depended on a distinction between homo- and hetero-. That gender-changing operation and other, less dramatic transgender behavior are blurring our neat borders should help us understand the very different Roman attitudes. Today you can have a lesbian who was born a man and a gay male who was born a woman or a male in prison who behaves in ways that to the outside world appear homosexual, but to the prison, the community does not, alongside the more traditional homosexual, bisexual, and heterosexual roles.
How Did the Romans See Gender?
Instead of today's gender orientation, ancient Roman (and Greek) sexuality can be dichotomized as passive and active. The socially preferred behavior of a male was active; the passive part aligned with the female.
"The relation between the 'active' and 'passive' partner is thought of as the same kind of relation as that obtaining between social superior and social inferior. - Malakos
But before I go further, let me stress: this is an oversimplification.
To Be an Ancient Roman Male in Good Standing
"… Walters makes a crucial distinction between 'males' and 'men': 'Not all males are men, and therefore impenetrable.' In particular, he refers to the special nuance of the term vir, which 'does not simply denote an adult male; it refers specifically to those adult males who are freeborn Roman citizens in good standing, those at the top of the Roman social hierarchy -- those who are sexually impenetrable penetrators'" Craig A. Williams' Bryn Mawr Classical Review of Roman Sexualities
"… since the concepts 'heterosexual' and 'homosexual' did not exist, but there does seem to be a high degree of correlation between the conduct of men identified as cinaedi and that of some men now labeled 'homosexuals,' though it must be appreciated that the modern term is clinical while the ancient one is emotional and even hostile, and that both have been imposed from outside." Richard W. Hooper's Bryn Mawr Classical Review of The Priapus Poems
To be an ancient Roman male in good standing meant you initiated penetrating acts of sex. Whether you did this with a female or a male, slave or free, wife or prostitute, made little difference -- as long as you were not on the receiving end, so to speak. Certain people were off-limits, though, and among them were free youths.
This was a change from the Greek attitude which, again to simplify, condoned such behavior in the context of a learning environment. The ancient Greek education of its youth had begun as training in the arts necessary for battle. Since physical fitness was the goal, education took place in a gymnasium (where physical training was in the buff). Over time the education came to encompass more academic parts, but instruction in how to be a valuable member of the polis continued. Often this included having an older male take a younger (post-pubescent, but still unbearded) one under his wing -- with all that entailed.
"Although later Romans sometimes asserted that homosexuality was imported from Greece, by the close of the 6th century B.C.E, Polybius reported, there was widespread acceptance of homosexuality Polybius, Histories, xxxii, ii." Lesbian and Gay Marriages
For the ancient Romans, who claimed to have adopted other "passive" behaviors from the ancient Greeks, free youths were untouchable. Since adolescents were still appealing, Roman males gratified themselves with youthful slaves. It's thought that in the baths (in many ways, successors to the Greek gymnasia), freedmen wore a talisman around their necks to make it clear their naked bodies were untouchable.