A distinction has to be maintained between the “permanence” of marriage and the “indissolubility” of marriage. “Marriage is permanent” means that it is a stable relationship, not subject to a term limit. A person cannot be ignorant that marriage is permanent and still have valid consent (c. 1096). “Marriage is indissoluble” means that neither spouse can do anything to dissolve the marriage bond itself. A person cannot exclude the property of indissolubility from his consent and still gave a valid marriage (c. 1101.2).
Since the law allows for separation of spouses due to infidelity, there has to be more going on than a mere “if you are unfaithful, then I won’t continue to live with you” in order for the marriage to be invalid due to an exclusion of indissolubility. The person would have to be of the mind that infidelity either itself dissolves the marriage bond or it gives the innocent spouse the right to dissolve the marriage bond. In addition to that, the person would need to have a motivation for excluding the indissoluble nature of marriage.
For example, a woman knew that the guy was a philanderer during the courtship and had good reason to think that he would continue to be so, even in marriage. She was of the mind that if he was unfaithful during the marriage, she had the right to divorce and marry again. It was her firm intention to divorce as soon as infidelity was discovered and she would thereby be free of this marriage bond. They had a child together so she also thought that getting married would be a better situation than simple cohabitation–so, it was worth a try.
Dan