III. The Apostolic precept about the marriage of the clergy based upon the marriage of Christ with the Church of which it is a figure.
For as the Apostle says that among other rules for election he shall be ordained bishop who is known to have been or to be “the husband of one wife,” this command was always held so sacred that the same condition was understood as necessary to be observed even in the wife of the priest-elect: lest she should happen to have been married to another man before she entered into wedlock with him, even though he himself had had no other wife. Who then would dare to allow this injury to be perpetrated upon so great a sacrament, seeing that this great and venerable mystery is not without the support of the statutes of God's law as well, whereby it is clearly laid down that a priest is to marry a virgin, and that she who is to be the wife of a priest is not to know another husband?
For even then in the priests was prefigured the Spiritual marriage of Christ and His Church: so that since “the man is the head of the woman,” the spouse of the Word may learn to know no other man but Christ, who did rightly choose her only, loves her only, and takes none but her into His alliance. If then even in the Old Testament this kind of marriage among priests is adhered to, how much more ought we who are placed under the grace of the Gospel to conform to the Apostle's precepts: so that though a man be found endowed with good character, and furnished with holy works, he may nevertheless in no wise ascend either to the grade of deacon, or the dignity of the presbytery, or to the highest rank of the bishopric, if it has been spread abroad either that he himself is not the husband of one wife, or that his wife is not the wife of one husband.
Source: Letters (New Advent)