One sage asked another: If two men sleep naked in the same bed, facing each other, they are forbidden to recite the Shema because each sees the other’s sexual organ. What is the rule if they sleep naked back to back, rear end to rear end? The answer: since they do not face each other’s sexual organs and do not see them, they are permitted to recite the Shema. The scholars wondered about this and noted: Even after their backs are turned, the cheeks of their rear ends still touch each other. They were answered that the cheeks are not considered nakedness and that one is permitted to recite the Shema. (This translation was prepared in accordance with Rashi’s commentary. The are many other strange and different commentaries, and much ink has been spilled on this matter.)
(Babylonian Talmud, Tractate Berachot 24a)