I’m not aware that divorce is a sin. The Church recognizes that there are conditions wherein a civil divorce is the only option available, e.g. a case of abuse.
I may be mistaken, but I thought that annulments could take place if it were proven that the spouse was not true or not committed to the vows they said. Therefore, the act of infidelity being in direct contrast to their vow.
If it could be established that the spouse never intended to be faithful at the time the marriage took place, maybe. If he or she sincerely intended to be faithful and then subsequently messed up, that wouldn’t establish that they were insincere in their initial vow.
We all fail to live up to our intentions all the time, unfortunately. Hopefully not by cheating on our spouses, but like every time we confess we have to intend to not sin in the future. But then, sooner or later, we do, and have to confess again. The flesh is weak.
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