In the last few ti morceaux I’ve dealt with the ministry of the Diocesan Tribunal, and in particular a few of the more-likely “grounds” found in marriage-nullity cases.
We’re considering the “grounds” of marital invalidity: certain defects can result in the invalidity of a marital union, if they are serious enough and certainly were present. In particular, an improper content of consent invalidates. When two people marry both have to intend to establish matrimony as God Himself defines this, for then every essential element and property of marriage will be included in the nuptial commitment. Last time I focused on “exclusion of children” as a possible cause of nullity, since the procreation and education of offspring is an essential “end” of marriage.
Another “essential” of marriage is nuptial faithfulness (also called “unity” in canon law). The Catechism of the Catholic Church in its art. 1646 emphasizes this well: “By its very nature conjugal love requires the inviolable fidelity of the spouses” [and] “The "intimate union of marriage demands total fidelity from the spouses and requires an unbreakable union between them.” Thus every person who marries must make a proper, irrevocable commitment to fidelity in the marriage. It’s never okay to pretend one has a “right” to infidelity, or think that one spouse can tolerate such behavior on either’s part.
May people mistakenly think that adultery would then be grounds for an annulment. No, not exactly. Sin, even the horrible sin of betraying a spouse in this way, can later happen in the life of someone who made the proper commitment at first. Yes, it’s unlikely, but it happens. In such a case that always mortal sin needs to be forgiven – both in confession of course and by the wronged spouse, even though the latter will take longer and surely be the more difficult task.
But what can be called a “pre-existing mindset” to reserve to oneself a selfish prerogative to act contrary to marital fidelity – much less think that one’s spouse has to accept this – does invalidate marital consent. In Christian marriage nothing justifies adultery, and when infidelity does happen in our day and age it almost always reveals not just human weakness but that a true personal commitment to marital unity was never made.
As I said in the last morceau, in matter of marital consent overarching selfishness invalidates. If the exclusivity of the union, or other essentials, are rejected from the outset, a bride or groom may appear to be marrying validly, but they’re not.