Rambam, Hilchot Isurei Biah 21:8

It is forbidden for women to rub against each other; it is among the “ways of the Egyptians,” about which we were warned in Leviticus 18:3 and about which our sages expounded, “What would they (the Egyptians) do? A man would marry a man, a woman would marry a woman, and one woman would marry two men. While the behavior is prohibited, one does not punish it with lashes because no specific biblical prohibition has been violated and no sexual intercourse took place at all. Consequently, such women are not prohibited from marrying a priest because of looseness, and neither is a [married] women prohibited from remaining with her husband[after a same-sex extramarital affair] because this behavior is not formally considered sex. It is, however, appropriate to punish such women with lashes imposed [not by the Torah but] by the court since they have violated a prohibition. A man ought to be exacting with his wife on this matter and should prevent her from associating with women known for this, not to permit those women to visit her nor her to visit them. [Translation by Rabbi Steven Greenberg]