We get the feeling that Myrtle Wilson is not an especially smart woman. Strung along by Tom, Myrtle is convinced that he loves her and would leave his wife for her if he could. The whole bit about Daisy being a Catholic and not believing in divorce is, as Nick points out, not remotely true.
Because she is unhappy in her marriage to George, Myrtle is drawn to Tom for certain specific reasons. George is passive, but Tom is controlling and authoritative. Myrtle puts up with Tom’s physical abuse because she equates it with masculinity – a quality that in her mind is lacking in her husband. She even yells at George, "throw me and down and beat me, you dirty little coward!"
Myrtle also adds to the novel’s themes of class and wealth. She insists that she married below her caste, that she believed certain things about George until they got married and it was too late (he borrowed a suit for the wedding, for example). Since Myrtle is quite obviously below the Buchanan’s class (yet another reason she goes for Tom), Fitzgerald ridicules her for insisting that she is above her husband.