How would someone know that?
There was an article about it back in '99 where Juanita Broderick claimed that Bill told her he was sterile.
"Broaddrick said that after the assault, Clinton told her not to worry about pregnancy because childhood mumps had rendered him sterile." - https://en.wikipedia.org/wiki/Juanita_Broaddrick#cite_note-rabin-10 (Original article is paywalled).