Section — Slater, Manual of Moral Theology
Section I. Adultery with Promise of Marriage
When husband or wife commits adultery with a third person and promises to marry that person after the death of the other spouse, the Church makes the adulterers incapable of contracting a valid marriage even after the first has been dissolved by death. The aim of the Church is to protect married people, to guard the sanctity of marriage, and to punish crime. A law, however, which restricts the liberty of marriage must be strictly interpreted, and so Doctors require the following conditions in the adultery and in the promise in order that the impediment may arise:
(a) The adultery must be real, formal on both sides, and complete. It must be real, or one at least of the parties must be united in a true, valid marriage. That fact must be known to both the adulterers, or else they are not guilty of formal but merely material adultery. If each party knows that the other is married and the other conditions are verified, there will be a double impediment between them. The adultery must be complete, so that it would be possible for it to produce its natural result in offspring.
(6) The promise, too, must be real, not fictitious, accepted by the promisee, absolute, not conditional, made with knowledge of the present marriage, and undertaking to contract marriage after the death of the other spouse. For one of the objects of the law is to remove the temptation to plot against the life of husband or wife.
Both the adultery and the promise must have place during the continuance of the same marriage, but it is immaterial whether the promise be made before, after, or at the same time as the adultery is committed.
p. ii, 226–227
This impediment of crime also exists between parties who have committed adultery with each other and attempted to marry during the lifetime of the spouse of one of them. The adultery must have the same qualities as in the preceding case, and the marriage must be really and truly attempted, not merely feigned. It is immaterial whether the attempted marriage precede or succeed the adultery. It is obvious that those will lie under this impediment who, after a civil divorce from a spouse, marry again and consummate the attempted marriage.
p. ii, 227