Whenever one partner cheats in a relationship, there is generally a time when each person is busy placing the blame on the other. The partner who finds out they have been cheated on is deeply hurt, and the cheater is looking for ways to place the blame on them rather than accepting it for their own behavior. It is a symptom of a deeper cause, so placing the blame on the partner who did not cheat is nothing more than covering up for bad behavior.
A partner who cheats can do it for many different reasons, but many of them line up excuses in the back of their mind because they know they will eventually be caught. Few of them will look honestly at their own behavior for wrongdoing, so their excuses tend to be anything they can define as a weakness or deficit in their partner. It has little or nothing to do with their partner, and the preparation of these excuses has more to do with avoiding responsibility or guilt for their own behavior.
By being prepared with a variety of excuses, a person who cheats is looking for a way to absolve their wrongdoing. They might eventually be able to convince themselves they are doing the right thing by cheating, and they could even see it as a way to justify helping their partner be a better person. It is a way for them to avoid the reality of taking responsibility for their own actions, and it should be seen by their partner in this light.
For those who have a cheating spouse or partner, being blamed for it often creates even more emotional pain. Rather than accepting the excuses, they should take a hard look at the fact their relationship is not working. They can remain within the relationship if the cheater is willing to make changes, but it should begin with them accepting the blame for their own actions.