Shabbos 65a: נשים המסוללות

The gemara relates that Shmuel's father did not permit his daughters to sleep together in the same bed. The gemara asks: Shall we say that this supports R. Huna? For R. Huna said: Women that commit lewdness with one another "מסוללות זו בזו" are unfit for the priesthood. The gemara answers that this was not Shmuel's father's reasoning. The reason he prevented them was so that they should not be accustomed to sleeping next to a foreign body [כי היכי דלא לילפן גופא נוכראה].

There is a very shtark kasha on a Rambam from this gemara. The Rambam [אסו"ב כא ח] says that if two women act inappropriately with each other it is subsumed under the category of מעשה ארץ מצרים  - the immorality of the Egyptians, from which we are warned by the Torah to distance ourselves. The implication of the Rambam is that it is an איסור דאורייתא. Why does the gemara neglect to mention this? The only concern voiced was that they would be forbidden to Kehuna. And the gemara's conclusion that his concern was that לא לילפן גופא נוכראה is also surprising. Why wasn't he concerned that unintentionally while sleeping they would be מסוללות זו בזו. The complete omission of this concern by the gemara implies that it is really not an איסור דאורייתא as the Rambam seems to say. 

Of course halacha limyse is like the Rambam and this Rambam is the source forbidding what is all too common in modern society... In the Halachic Medical Encyclopedia of Dr. Steinberg Shlita [Vol. 4 page 717] he cites numerous sources which discuss the severity of the sin.

עי' בספר אבני חפץ עמ' קנ מש"כ בזה.