It's complicated.

This is going to be a long answer with several parts. I'm going to cover both Greek and Roman attitudes toward women's homoeroticism, since it is not possible to understand the Roman attitudes without understanding the Greek attitudes that came first.

Unfortunately, while references to men's homoerotic attraction and relationships are absolutely ubiquitous throughout the surviving ancient Greek and Roman sources, women's homoerotic attraction and relationships are very poorly attested. To say that the primary sources on this subject are scant is an understatement. This paucity of evidence is mainly the result of the fact that nearly all the surviving ancient sources were written by men who were generally not interested in writing about anything women did among themselves when there were no men around.

Based on the admittedly very few sources that we have, though, homoerotic attraction and relationships seem to have been relatively common and not heavily stigmatized among Greek women in the Archaic, Classical, and Hellenistic Eras. Attitudes toward women’s homoeroticism in the Roman world, by contrast, seem to have varied drastically. Roman-era sources variously portray women’s homoeroticism as a degenerate Greek perversion, as something that should amuse and titillate male audiences, as an absurd impossibility, as an allegation against which a woman’s reputation must be defended, and, finally, in some cases, something that should be accepted as normal.

A general note about the ancient Greek and Roman conception of sexuality

Before I discuss the ancient sources, it is important to clarify that modern concepts of sexual orientation are not directly transferrable to ancient Greece and Rome. The ancient Greeks and Romans generally had no concept of "sexual orientation" as an innate identity based on which gender or genders a person was erotically attracted to. There are no words in Ancient Greek or Latin that are directly equivalent to the modern English terms "gay," "lesbian," "bisexual," or "straight."

The Greeks and Romans, of course, recognized that some people are more erotically interested in one gender over another, but they generally did not think of this preference as a fixed, innate aspect of a person's identity. Instead of thinking about sexuality in terms of the gender or genders a person was attracted to, the Greeks and Romans usually thought of it in terms of the role that a person took during sex.

The normative Greek and Roman conception of sexuality was extremely phallocentric and entirely focused on penile penetration. Normatively speaking, the Greeks and Romans believed essentially that sex was supposed to be an activity in which a free adult man was supposed to prove his masculine superiority by using his erect penis to dominate an innately inferior person, such as woman, adolescent boy, or enslaved person, by penetrating them in one of more of their various orifices. To penetrate was seen as innately masculine, glorious, and superior, while to be penetrated was seen as innately feminine, shameful, and inferior.

In line with this conception of sexuality, it was seen as completely normal and acceptable for a free adult man to have sexual relations with women, adolescent boys, and enslaved people of any gender, as long as he was always the one penetrating his partner and never the one being penetrated. Free adult men who were suspected of taking the penetrated role in intercourse with another man or other men were widely mocked. People would call them degrading and shameful names, such as κίναιδος (kínaidos), βάταλος (bátalos), εὐρύπρωκτος (eurýprōktos), and μαλακός (malakós) in Greek or cinaedus, pathicus, and mollis in Latin.

As a result of this, with only a few noteworthy exceptions, whenever a Greek or Roman man tried to imagine two women having sex, he always imagined one partner wearing a strap-on dildo to penetrate the other partner in exactly the same way that a man with a penis would typically penetrate his sexual partners. The very idea that there might be any form of sex that doesn't involve a partner with a penis penetrating the other partner seems to have rarely even registered to them.

This may partly explain why there are many ancient sources that mention husbands being concerned about their wives having adulterous sex with other men, but only one that I am aware of (which I will discuss shortly) that expresses concern about wives having adulterous sex with other women. Quite simply, for most Greek and Roman men, any kind of sex that didn't involve a man with a penis penetrating someone wasn't real sex and therefore a woman having sex with another woman was not real adultery and not something that men really needed to worry about.

Sappho's homoerotic poems

As most readers are probably already aware, the Greek poet Sappho (lived c. 630 – c. 570 BCE), who lived on the island of Lesbos just off the west coast of what is now Turkey, composed poems in which she speaks very openly about homoerotic attraction and even implied sex among women. These poems are mostly written from the perspective of herself as a character.

Her most famous poem is probably her Fragment 31, which is sometimes known as "Phainetai Moi" after its Greek incipit. It has been preserved nearly in its entirety through quotation by the Roman-era writer Longinos in his treatise On the Sublime 10.1–3. In the poem, the character Sappho vividly describes her intense feelings of envy and longing when she sees the woman she desires consorting with a man:

“φαίνεταί μοι κῆνος ἴσος θέοισιν
ἔμμεν᾽ ὤνηρ, ὄττις ἐνάντιός τοι
ἰσδάνει καὶ πλάσιον ἆδυ φωνεί-
σας ὐπακούει”

“καὶ γελαίσας ἰμέροεν, τό μ᾽ ἦ μὰν
καρδίαν ἐν στήθεσιν ἐπτόαισεν·
ὠς γὰρ ἔς σ᾽ ἴδω βρόχε᾽, ὤς με φώναι-
σ᾽ οὐδ᾽ ἒν ἔτ᾽ εἴκει,”

“ἀλλ᾽ ἄκαν μὲν γλῶσσα †ἔαγε†, λέπτον
δ᾽ αὔτικα χρῶι πῦρ ὐπαδεδρόμηκεν,
ὀππάτεσσι δ᾽ οὐδ᾽ ἒν ὄρημμ᾽, ἐπιρρόμ-
βεισι δ᾽ ἄκουαι,”

“†έκαδε μ᾽ ἴδρως ψῦχρος κακχέεται†, τρόμος δὲ
παῖσαν ἄγρει, χλωροτέρα δὲ ποίας
ἔμμι, τεθνάκην δ᾽ ὀλίγω ᾽πιδεύης
φαίνομ᾽ ἔμ᾽ αὔται·”

“ἀλλὰ πὰν τόλματον ἐπεὶ †καὶ πένητᆔ

Here is my own translation:

“That man seems to me equal to the deities,
the one who sits across from you
and, beside you, listens to
your soft speaking,

and your laughing lovely: that truly
makes the heart in my breast pound;
for, as I look at you briefly, it is no longer
possible for me to speak,

but my tongue has broken, and immediately
a slight fire runs beneath my skin,
I cannot see anything with my eyes,
and my ears are buzzing,

†a cold sweat pours down me,† and a trembling
seizes me all over, and I am sallower than grass:
I feel as if I’m not far off dying.

But all things must be endured, since †even a pauper†”

Here the extant text of the poem abruptly leaves off.

(This answer is continued in the comment below.)