64 We have stated what is according to the law, let us state in addition what is according to reason. But first we must notice that not only has the Apostle laid down this rule concerning a bishop or priest, but that the Fathers in the Nicene Council added that no one who has contracted a second marriage ought to be admitted among the clergy at all. For how can he comfort or honour a widow, or exhort her to preserve her widowhood, and the faith pledged to her husband, which he himself has not kept in regard to his former marriage? Or what difference would there be between people and priest, if they were bound by the same laws? The life of a priest ought to excel that of others as does his grace, for he who binds others by his precepts ought himself to keep the precepts of the law.
Source: Letters (New Advent)