Loyalty does not consider change in circumstances. If I am loyal to my partner and my partner meets with an accident that disfigures him/her badly, does not mean that I go away to somebody better looking. If I commit to someone about my relationship and that person goes broke after sometime, does not mean that I will look for richer persons. Every situation that comes tests loyalty. In some countries the law allows you to break your marriage owes if your partner goes lunatic. But many loyal people don't break the relationship even at that time.
Every relationship rests on trust and truth. If the relationship began with lies, it can never sustain. The foundation is of lies. How truth can grow from the seed of lie? The first need or requirement of going in a long-term relationship is to be truthful in all respects. Fooling others into believing untruths is a sin.
Once you have decided to and committed loyalty, you should never break it under any circumstances. There can be no excuse for that. By doing any such act, we hurt the other partner who believed in us, and we hurt ourselves by becoming an unfaithful and unreliable person. We lose our self-esteem totally after that. Once that happens, the path is only downhill, all the way to hell. Love and loyalty go together.How can we be disloyal in love? That is an impossible thought.
To love and to be loved is a blessing. To break that by being disloyal is sinning against goodness and God. If by any chance one does an act of disloyalty, one must immediately confess and ask for forgiveness. Will the relationship ever remain same after this act of disloyalty? No. Unless one is lucky to have an extra ordinary partner, the relationship will never remain the same. Better to be a loyal and truthful person always.