One of the things that is implicit in Boehringer's analysis, but not (yet) stated overtly (perhaps because she assumes her readers are aware of it?), is that there is a major shift in the development of "gender categories" between the earlier Greek evidence and the Roman evidence. Under Greek pederasty, the erastes and eromenos took on categorically different roles in the relationship, but they were not viewed as inhabiting distinct life-long identity categories. The eromenos is expected to participate in the relationship as part of the mentoring necessary to become an adult male citizen, not because he has a specific desire to be a "passive" partner in sex. He is, in turn, expected to become an erastes himself. And despite some hints of a pederastic element in f/f relations, the primary model is a different one: two equal partners in the relationship.
But the Roman sexual system established the possibility of a free adult man who, due to some inherent nature, had a lifelong attraction to taking a "passive" role in sex. Within Roman philosophy, this made him a less virtuous man, but it was an identifiable and defined category that was distinct from those males who might be forced into being the "passive" partner of another man due to their dependent social status. Similarly, the Roman sexual system had a role for an assigned-female person whose sexual desires and activities were categorized as masculine, whether or not that person had female partners. This was not an aspect of the Greek depiction of women in same-sex relations (to the extent that we have evidence). Greek myth did have a "stock type" of women who participated in male-coded activities, such as the followers of Artemis, but that element was not conceptually tied to certain sexual preferences.
The book will, no doubt, get into this development in more detail as the chapter goes on. But it's useful to keep in mind that this distinction in a Roman context is sometimes anachronistically projected onto Greek culture, as when Sappho is later portrayed as "masculine."
Boehringer, Sandra (trans. Anna Preger). 2021. Female Homosexuality in Ancient Greece and Rome. Routledge, New York. ISBN 978-0-367-74476-2
Chapter 3d: The Roman Period - Sexual Satire: Tribades - Phaedrus
While earlier references to f/f relations focused on emotions, with the start of the Common Era, Roman literature introduces different attitudes. The category of “tribade,” although derived from the Greek word “tribas” (from “tribein”, to rub), has its earliest surviving mentions in Latin texts. It was clearly in use previously as it appears in multiple texts at a similar era.
The fables of Phaedrus were inspired by those of Aesop, being short stories with a moral ending. One of them provides a comic “explanation” for the existence of certain sexual types: molles mares and tribades. The story tells that when Prometheus was in the process of creating human beings out of clay, one day he got drunk with Bacchus after a session of creating genitals, and accidentally put female genitals on male bodies and vice versa, resulting in “perverted pleasures” (pravo gaudo). This follows a separate fable of Prometheus in which he is said to have made male genitals out of the same material as women’s tongues, explaining their “similar obscenity”.
Although the contextual meaning of “tribades” cannot be derived from pre-existing examples, “molles” is known from other contexts. The literal meaning “softness” was applied to men whose sexual or gendered behavior differed from the norm in specific ways. Along with “impudicus” and “pathicus” it indicated traits that were considered feminine, with the extreme being the “cinaedus”. These terms covered a range of behavior involving dress, grooming, and speech, but also taking a passive role in sex including, but not limited to, enjoying being penetrated. So the “molles mares” are presumably the set of Prometheus’s creations that have the superficial appearance of men but are essentially feminine. Thus we have a connection between sexual desire and a category of men defined by something other than biological sex.
The text is not specific whether the superficial sexual category (i.e., the one people are assigned by society) is the one corresponding to the genitals or to the body they have been attached to. If one takes the ordering of the description in the text as parallel, then tribades are those who have female genitals on male bodies, and molles have male genitals on female bodies. Some historians have interpreted the reverse, that it is the genitals that drive sexual desire, therefore the molles have (female) genitals that want to be penetrated, while the tribades have (male) genitals that want to penetrate. This would connect the latter with the image of the tribade with an enlarged clitoris. The second interpretation would suggest that there should be other references to molles as having feminine genitals (given the numerous textual references to them) but this doesn’t appear to be the case. (Boeheringer discusses this question at length.)
So if we return to reading the genital substitutions in parallel order, with the genitals marking the socially assigned sex and the “body” representing the “orientation”, then the new category of tribas must represent a physiological female whose desires and social actions are coded as male.
For both the mollis and the tribade, the “perversion” involved is not homosexuality as modernly defined, but taking pleasure in something inappropriate to one's sex. [Note: as Williams and others have noted, the mollis and his lover do not belong to the same sexual category – the same “gender identity” if you will - because his partner is an active/penetrative man, or even in some cases a “active” woman. I other references to tribades, we see that the category is not exclusive to women to take a "male" sexual role with other women, but can also include women to take an active/penetrative role with a male partner. When historians talk about classical Roman society not having concepts that correspond to homosexuality and heterosexuality, this is what they mean: not that Roman society didn't recognize the phenomenon of persons of a particular biological sex engaging in sex with other persons of the same biological sex, but rather that their conception of those relations was not organized around seeing both partners as belonging to the same definable category.]
The molles and tribades are not placed in a single conceptual category on the basis of some shared attribute, such as "having the behavioral nature of the other sex". There is also no indication in this text of the later motif of tribades as having a phallic clitoris. And despite the joke in a previous fable about penises and women’s tongues being made of the same material, there is no clear indication that he is implying cunnilingus between women. In all Phaedrus’s discussions of sexual immodesty or depravity, the focus is on the acts of an individual with respect to their assigned social/gender role, with no consideration of the nature of their partner. However the fable does suggest an “essentialist” view of sexual preferences – that certain people behave sexually and socially in certain ways due to their inherent nature. But the categories defined by this nature do not correspond to modern categories of sexual orientation. They do correspond to categories but to different categories than modern ones.
There are vague similarities here to the myth of the "two-bodied persons" in the Symposium, but both the nature of the resulting categories and the attitude of the narrator to these categories is different. Phaedrus is making fun of origin myths, at the same time that he’s mocking effeminate men and masculine women as being the result of a drunken mistake. But without the context of further references to tribades in Phaedrus's time (as we have for molles) we can’t tell whether the depiction here is a reflection of popular attitudes, or a comic exaggeration, or a complete invention.