Continuity mistake: In the scene where Justine's husband needs 'help' at the doctor's office, Justine is wearing a flannel shirt and overalls. Then, when she's back at work, she's suddenly wearing a multicolored sweater briefly for one scene (which is what she wears two days later during the day she talks to Holden's parents), then at the end of the day she's suddenly wearing the same flannel shirt and overalls. (00:57:45)
Continuity mistake: In one scene when Justine and her husband are in bed together, after the affair with the kid has already started, you see her back and her husband's face. Her hair is behind her ear without any loose threads over her ear, then when it switches, back of him to front of her, her hair is in front of her ear. It keeps going back and forth like this throughout the whole scene.
Answer: I think she wanted to put the whole situation behind her. When Phil asked if she cheated with the security guard from work, she let him believe it. She was able to convince him that the baby was definitely his, not the guard's. Telling him about Bubba would only make the situation worse. Also, Bubba had seen her with Holden, so he could try and convince Phil that she was lying about the security guard. It would create a bigger mess. She decided to accept her life as it was.