I know it could be argued that if this happens, the outside partner is the one they are called to marry in the first place, never mind the injustice done to the abandoned fiance.
It could go either way, but in the sacrament of matrimony, aren’t graces given to remain faithful to one’s spouse, even when “more attractive opportunities manifest themselves”? And does not the evil one, who hates grace and the sacraments, seek to destroy weak marriages so that the sin of adultery will be committed? What better way than to exploit the spouse’s attraction to someone else, and possibly to “egg on” one’s distaste for the other spouse’s shortcomings? (We all have shortcomings.) He knows our weaknesses and knows “what buttons to push”. Engagements and betrothals are not grace-filled sacraments. Matrimony is.
