Commentary for Kiddushin 132:8

Rava now shows how he can prove that in sexual matters, two witnesses are always necessary. It is not sufficient to have one witness plus the silence of the accused. In the case of the blemish, where one witness is sufficient, it must be that the priest himself does not protest. Otherwise, one witness would not be believed. By analogy, in the case of the woman being a divorcee, there must be two witnesses, even if the priest is silent. The priest would not be disqualified if there was one witness and he was silent. This is proof for Rava.