I arranged a matrilineal marriage with some interesting claimant. By the time kids came of age, the husband-to-be went up in the line of succession and now his liege will not accept marriage proposal because the guy is 'too high in the line of succession'. Neither will he suggest it himself. Can I somehow force the marriage? Can I exit the betrothal without getting 'dishonored betrothal commitment' in my face? Can I get the dishonor on the other guy? As of now my bride is doomed to die childless and not benefit the house.
Well, from my own experience it's only matter of time before AI liege will suggest marriage himself, even if current opinion and succession line situation indicated it is impossible. It happened to me multiple times - most notable situation playing as a Duke of Greater Poland, having my daugther married matrilineally to HRE heir, becoming empress in the same year.
EDIT: Actually I had situation when bethrotal got locked due to husband-to-be becoming knight of military order. I've just broken the bethrotal, it's better to lose some opinion and try another bethrotal instead of losing benefit of marriage on one of your family members.