Here is the official teaching, direct from the Vatican website:
vatican.va/roman_curia/po…emecum_en.html
Quote:
13. Special difficulties are presented by cases of cooperation in the
sin of a
spouse who voluntarily renders the unitive act infecund. In the first place, it is necessary to distinguish cooperation in the proper sense, from violence or unjust imposition on the part of one of the spouses, which the other
spouse in fact cannot resist.46, 561).] This cooperation can be licit when the three following conditions are jointly met:
- when the action of the cooperating spouse is not already illicit in itself;47
- when proportionally grave reasons exist for cooperating in the sin of the other spouse;
- when one is seeking to help the other spouse to desist from such conduct (patiently, with prayer, charity and dialogue; although not necessarily in that moment, nor on every single occasion).
The “proportionally grave reason for cooperating in the
sin of the other
spouse” is usually understood in terms of keeping the marriage from falling apart. I hope this is helpful.
Fr. Vincent Serpa, O.P.