Butrica, in his article "Some Myths and Anomalies in the Study of Roman Sexuality" (in Same-Sex Desire and Love in Greco-Roman Antiquity and in the Classical Tradition of the West) writes

Though she claims that lesbianism was regarded as “criminal,” Cantarella has to admit that not a single law addressed it (see also Dalla, 1987, pp. 215-21). Her claim that a married woman who had relations with another woman committed adultery (p. 214) seems inherently unlikely, given that nothing of the kind is recorded in the Digest, for example, or mentioned in connection with Augustus’ law criminalizing adultery, or with the later repressions; and it turns out to be based on the punchline to Martial 1.90, where adulterium is used humorously in reference to a woman who penetrates other women with a dildo as a “virtual” man. The explanation for this neglect under Roman law is surely not far to seek: lesbianism cannot lead to unwanted pregnancy and therefore need not be prohibited by an authority whose principal goal is to preserve the integrity and indeed to maintain the existence of certain social classes. To judge by Roman law, heterosexual women represented an infinitely greater threat than lesbians.

Note, however,

Seneca the Elder's discussion of two tribades discovered together in bed by the husband of one of the women, who immediately kills both of them, illustrates the danger of homoerotic activity for married women, even though Seneca's account is fictitious.

(Brooten 1996:109)