Torah Temimah points to Berachos 21b / 22a (R’ Yehoshua ben Levi: a ba’al keri is forbidden in words of Torah by semichut to the day of standing at Chorev).
English: R’ Yehoshua ben Levi said: From where do we know that a ba’al keri is forbidden in words of Torah? As it is said “and make them known to your children,” and adjacent to it “the day that you stood before Hashem your God at Chorev” — just as there ba’alei keri were forbidden, so here ba’alei keri are forbidden.
On the daf / in context: The semichut equates the state required for the original reception of Torah with the state required for subsequent study and teaching.
Type of Learning
Semichut ha-parshiyos — “vehodatem” joined to the day of standing at Chorev yields the prohibition for a ba’al keri.
What Is Learned
A ba’al keri is barred from words of Torah by the same standard that applied at the original revelation at Sinai.