Fr. Rocky: The simple answer is: Pray. Smile. Forgive. But the more detailed answer follows.
Your question is fundamentally about cooperation in evil. So, the question is asked, “Can a person in good conscience ever cooperate in evil?” The answer is “yes,” but only for proportionate reasons. In such cases the cooperation must be under protest and indirect.
In your case, since you have a well-formed conscience, it would be a serious sin for you to use contraception (pills or devices or sterilization). If your husband insists on using contraception, then he has to be the contracepting person – not you – and in this case, he commits the sin, not you.
The Church, in her pastoral wisdom, addressed this case years ago in the papal encyclical Casti Conubii, written by Pope Pius XI and promulgated Dec. 31, 1930. In that papal document he wrote: “Holy Church knows well that not infrequently one of the parties is sinned against rather than sinning, when for a grave cause he or she reluctantly allows the perversion of the right order. In such a case, there is no sin, provided that, mindful of the law of charity, he or she does not neglect to seek to dissuade and to deter the partner from sin” (No. 59).
If the husband uses a condom during marital relations, he commits the mortal sin, not the wife. She is being “sinned against rather than sinning.” She can only cooperate in such an evil when “for a grave cause she reluctantly allows the perversion of the right order.” What would be such a “grave cause”? The husband might leave her or go somewhere else and commit adultery and thus destroy the family. If she were to cooperate, it must be “reluctantly,” and she must pray for him and continue to try to change his heart.
Fr. Francis J. Hoffman, JCD(aka "Fr. Rocky") is a priest of Opus Dei and Executive Director of Relevant Radio. His "Question and Answer" column appears in Our Sunday Visitor’s The Catholic Answer Magazine.