This one is extremely simple to explain - if you or your partner have to revert back to cheating, that means that the relationship has entered into the highest possible level of "toxicity." If you or your partner has cheated, most experts would say that there is no way to salvage a relationship like that, as all trust has already been lost, although there is always a small chance of improving things.
The problem is, cheating breaks the bond that holds the relationship together, which for many people means that the relationship is useless after one of the partners has cheated. Some say that two people can stay together even after cheating, but that would require a lot of time, effort and sacrifice in order to work things through - without any guarantee that things are going to work out.