Okay, so things with Alice get pretty weird—we're not going to deny that. That being said, we don't think you should judge her too harshly for her actions. Like many other characters in the novel, Alice has been shaken to her core by the pressure to have children.
As a woman, Alice has no choice but to face this pressure head-on. She tells Judd that she takes "a drug to make [her] ovulate" but "Paul won't even get his sperm tested" (31.20). It's easy for Paul to avoid the problem, but Alice doesn't have that luxury. With that in mind, it's less shocking that she made an irrational decision to turn to Judd.
Although he's not able to resist her advances, Judd reminds how important it is for her to preserve her marriage. Judd actually understands what she's going through—he was in Paul's shoes after Jen's miscarriage. Like Paul, Judd wasn't there for his wife during that trying time and she ended up looking elsewhere. In many ways, Judd does for Paul what he wished Wade would've done for him: tell the cheating wife to go back to her husband. Good work, Judd.