Married Catholic clergy do not take a vow or make a promise to be continent. We make a promise of conditional celibacy in that if our wives pre-decease us then we must remain celibate and cannot remarry.
If Deacon Cornelius is making a promise to be continent after ordination, then that is entirely of his own initiative - and all the more praiseworthy because of it. He would have to have his wife’s agreement in this, of course.
If they subsequently decided that they could not live continently, he would not be breaking any of the Church’s laws, therefore, it would be no more sinful than breaking any other promise. However, the prospect of breaking a promise we make to God should make any of us pause before making such promises without due care and consideration.
Forgive my ignorance, why is a married man's promise to be continent praiseworthy? I can only see it as a good thing if his wife hates relations, otherwise, I think he's being unfair to her. Of course, I'm not in their marriage, but neither is anyone else, so why announce their decision to the world?