A woman may not sexually arouse herself with another woman, because sexual desire must be reserved for the sacred love between husband and wife and for fulfilling the mitzvot of ona and procreation. The Torah admonishes, “You shall not copy the practices of the land of Egypt where you dwelt…nor shall you follow their laws” (Vayikra 18:3). The Sages explained (Sifra ad loc.) that the “practices of the land of Egypt” are “a man marrying a man, a woman marrying a woman, and a woman marrying two men” (MT, Laws of Sexual Prohibitions 21:8; SA EH 20:2).
Some Tanna’im and Amora’im maintain that a woman who is intimate with another woman is considered a zona, who may not marry a kohen. The Torah restricts kohanim from marrying certain women: “They shall not marry a zona or ḥalala, nor shall they marry one divorced from her husband. For they are holy to their God” (Vayikra 21:7). However, in practice the halakha does not follow this opinion. While lesbian sex is considered promiscuous, a woman who engages in it is not considered a zona, and thus may marry a kohen. Certainly, a married woman who engages in it is not forbidden to her husband (as would be a woman who commits adultery), since this activity involves no penetration in the way that a man penetrates a woman (Yevamot 76a; Rambam ad loc.).
The simple understanding is that women who are sexually intimate with one another transgress a Torah prohibition. This is implied by the possibility that such behavior is enough to disqualify a woman from marrying a kohen. This seems to be the opinion of Rambam as well. However, there is no punishment by lashes for this sin because there is no specific negative commandment against it (MT, Laws of Sexual Prohibitions 21:9). This is also the opinion of Mabit (Kiryat Sefer ad loc.) and Levush, EH 20:2. In contrast, Kiryat Melekh Rav 2:26 states that there is a Torah prohibition on such activity only in the context of a long-term relationship, something resembling marriage. Prisha explains that, according to Tur, the prohibition is always rabbinic (20:11).