These are an interesting two concepts to consider within the book Erasure! First let's look at the definitions, and then we will attempt to see their forms within the novel.
In my opinion, infidelity has always had a really interesting definition because it doesn't always mean a sexual affair outside of marriage. In fact, infidelity is more of a breach of the contract between the spouses. It implies that the two have agreed to a certain amount of (or kind of) emotional and/or sexual and/or mental intimacy. As a result, the two have promised not to enter into the same kind of intimacy with another. Infidelity, then, doesn't always have to involve sex. It can be infidelity in the emotional or mental regard, instead. However, it should be said that infidelity (at least in our society) does imply adultery (which is sexual relations outside of marriage).
Misogyny has an interesting definition as well. It is, quite simply, the dislike of women. It often presents itself in discrimination of women in society. Misogyny can even lead to violence against women. Probably the best explanation of this term is by the sociologist Alan Johnson:
Misogyny is a central part of sexist prejudice and ideology and, as such, is an important basis for the oppression of females in male-dominated societies. Misogyny is manifested in many different ways, from jokes to pornography to violence to the self-contempt women may be taught to feel toward their own bodies.
In regard to Erasure, infidelity becomes a prominent part of the novel due to Ellison learning the truth about his past. Ellison has always felt different than his siblings, and there is a simple reason why: infidelity. When Ellison's father was in the Korean War, he had a sexual affair with a woman there. The woman in Korea, however, decided against being with Ellison's father. This is the only reason Ellison's dad returned to his wife. Ellison was conceived when his dad reunited with his wife AFTER the affair. Therefore, infidelity in Erasure does take the form of a breach of sexual intimacy in the actions of Ellison's father.
Analyzing the idea of misogyny is interesting when thinking about Erasure. In my opinion, misogyny is represented through Ellison's two siblings, Bill and Lisa. As a homosexual, Bill feels lots of family pressure to deny his sexual preference in order to honor both conformity and society. As a result, Bill has definite negative feelings towards women: a gender Bill never really wanted to be united with. In regard to Lisa, her life is taken by someone who thinks he is doing society a favor by removing Lisa from the world. "Lisa’s life is erased by an ideological terrorist who thinks that he is saving lives by taking lives." So Lisa is directly "removed" by misogyny.