- Willful consent. - if willful consent to attempts at the marital act was not given, that is rape, and I agree that at least one party would not be culpable.
With respect I think you are confusing consent to engage in a sexual act with consent of the will with respect to the ability to resist engaging in sin in spite of our better judgment. Moreover in a marital context, sexual consent is often implied and not explicit. But it is not related to consent of the will in overcoming sin, especially habitual sin.
The CCC says with respect to masturbation, for instance (my bold):
To form an equitable judgment about the subjects’ moral responsibility and to guide pastoral action, one must take into account the affective immaturity, force of acquired habit, conditions of anxiety, or other psychological or social factors that lessen or even extenuate moral culpability.
It is quite possible, and IMHO, quite likely, that in a long-standing second (non-valid) marriage, that the “force of acquired habit” of conjugal relations may in fact mean that the couple, mutually, are giving into their own, mutual, weakness. That is, their will isn’t fully consenting to engaging in what the Church considers an illicit sexual relationship, for the same reason that the Church recognizes that force of acquired habit can mitigate culpability for masturbation. Surely if it’s good for one sin, it’s good for the other, since the “habit” in this case is almost identical, the need for sexual gratification.
This is what I’m driving at with “full consent of the will”. A couple may have been in the invalid second marriage for so many years, that “force of an acquired” sexual habit maybe difficult to overcome by willpower alone for either or both parties. If the desire is to move their souls closer to God, and assuming the first marriage was valid and cannot be annulled, then withholding the sacraments from them may in fact be making the sacraments a “prize for attaining perfection” rather than as a medicine to help them attain Christian perfection.
Now, the following is my personal opinion, and I know I will get blasted for it as being heretical. But Christ as I mentioned elsewhere, was spat on, whipped, had a crown of thorns driven into his skull, was nailed to a cross, and was pierced by a lance, left to die of exposure. He did that for our sins, so that we can be reconciled with God; He did not do that to put roadblocks on peoples’ reconciliation. I do think He can handle an invalidly married couple, who want to come closer to Him, receiving Him. In other words, it’s a matter of conscience, and ongoing conversion through reconciliation and the graces of the Eucharist.
It is not up to us to put limitations on what God can, or cannot do or on who should or should not approach him. The Church does have the right to ensure that people do approach Him in faith, and with the proper mindset. But that proper mindset does not mean being held to a standard of perfection that does not apply to convicted murderers, or for that matter habitual masturbators. None of us are saints; and that proper mindset is something that I think needs to be determined in the confessional, not in sort of “one size fits all” manual. None of this requires refutation of the doctrine of indissolubility of marriage.