Chapter 72:12-13
The Sanctity of the Sabbath
12. It is a Mitzvah (1) for each person to wash his face, hands, and feet
in hot water on Friday. If possible, he should wash his entire body, and
immerse himself in a Mikvah (ritual bath).
13. It is forbidden to bathe (naked) with either one's father,
father-in-law, mother's husband or sister's husband (2). In places where it
is customary to cover one's nakedness in the bath house, [bathing with
these relatives] is permitted (3).
A student should not bathe together with his Rebbe (teacher) (4); however,
if his Rebbe needs his help, it is permissible (an exception which does not
apply to the aforementioned relatives).
(1) Part of the mitzvah to honor the Shabbos.
(2) It is prohibited for a male to engage in sexual fantasizing; one reason
for this prohibition is as a safeguard against experiencing seminal
emissions, whether they be during the day or while asleep at night ( all
seminal emissions outside of sexual intercourse with one's wife, are
prohibited; any act, observation or thought that may lead to conscious or
unconscious seminal emissions is also prohibited). This rabbinical
enactment against bathing with particular relatives is a safeguard against
sexual fantasizing.
(3) The Aruch HaShulchan (Even HaEzer 23:8) states that he is unsure
whether it is permissible for a father to bathe with his son who is under
13 years old.
(4) To do so would represent a lack of "Derech Eretz" (respect).
