If he has a bad temper, then one facet of his personality is flawed. But there is so much left to love. And if she is immature, then he can still appreciate all the other wonderful things about her. Everything the adulterer says is suspect. It is not merely that he possesses imperfections, rather there is nothing about him to love because there is nothing about him which is authentic.
Nothing is real enough as to engender loyalty and devotion.
To be sure, they may still be trustworthy in other areas, just as someone who cheats in business may still be faithful to his wife. But what is important in this situation is not the way things really are, but rather the perception which the wife will have. And she thinks him to be a scoundrel. Even if he apologizes, the spouse will not be able to forget that just last week he was telling his wife how much he loves her, but at the same time running around behind her back with someone else. This loss of trust is not like having a bad temper or being miserly.