Say one commits a serious sin, e.g., cheating on one’s significant other. Say further that one wants to confess such a sin, but can one do so without first admitting it to the injured party? Surely the effective misrepresentation of oneself as a good candidate for matrimony is itself a serious sin, right?
A priest cannot require of a penitent any action that would reveal the contents of the sacramental confession to outsiders, including those who have been injured. So, if a person confessed cheating on his “significant other” to a priest in the confessional, the priest cannot condition absolution upon confession of the deed to the “significant other.”
That said, honesty between those who are discerning marriage to each other is critical to the success of the married life. If there are commitment problems now, prudential judgment – to say nothing of justice – should caution that the “significant other” should know about this before the marriage when there is still time for both to determine whether they are able to give to each other lifelong fidelity. The penitent cannot be required by a priest in the confessional to inform the injured party of the problem, but the priest may certainly encourage it out of concern for the success of a future marriage.