Section — Noldin, Summa of Moral Theology
Section 3. Attempted Marriage.
493. Because of Attempted Marriage these are irregular: those who have dared to attempt marriage, or to place only a civil act, while they themselves are bound either by a matrimonial bond, or by sacred order, or by religious vows even simple and temporary; or those who have attempted marriage with a woman bound by the same vows or joined in valid marriage.1
a. It does not matter whether the diriment impediment existed on the part of the man or of the woman; whether they attempted marriage before the Church or placed only a civil act.
b. In all cases which the cited canon enumerates, the marriage is invalid because the same marriage is opposed by either a marriage still existing, or sacred order, or solemn vow. Yet in that case in which a man bound by simple vows, or an entirely free man, contracts marriage with a woman bound by simple vows, the marriage is indeed valid, but irregularity is nevertheless contracted, even if the simple religious vows are temporary only.
1 Cn. 985, 3.