Parashas Metzora · Passuk 15:18
Chabad.org — English translation + Rashi
Torah Temima points to Niddah 41b.
English: “And they shall wash in water and be impure” — R’ Shimon said: What does this come to teach? If regarding one who touches shichvas zera, that is already stated; rather, because it is impurity of a hidden place, and impurity of a hidden place does not ordinarily convey impurity — except that here it is a decree of Scripture.
Scriptural exception for beis haseetarim
Chazal explain that this verse specially imposes impurity despite contact occurring in a hidden place.