The thing is, “deliberate consent” means more than “choosing the sinful act”. They must do so on its own, for its own sake, desiring the sin.
In the case of an irregular marriage, some cannot choose continence (i.e., complete cessation of marital relations). If continence would create a strain on the new (irregular) marriage such that children of that new marriage would suffer (e.g., if the couple should split because of the attempt at continence), then the choice to commit the sin wouldn’t be from “full consent”, but in a spouse’s or spouses’ grave fear for the consequences of not allowing marital relations to continue.
In that case, the pastoral approach might counsel prudence, over time, allowing the couple to come to an understanding of how such a solution would be the best approach for them. Gradually, over time, having been told of the objective sinfulness of their actions, a pastor would hope to bring them to a lived understanding (rather than just an intellectual understanding) of the situation. In such a way, the hope would be to transform them into avoiding that sin.