She'd Researched Infertility
Julia had assumed Mark's distant behavior was her fault. The fact that they might not be able to have children devasted her, and maybe it had driven him to an affair. Mark didn't want her to blame herself...
She Knew She Couldn't Stay
Mark insisted his behavior wasn't her fault, but he wanted to work on things. She knew in her heart that she couldn't do that, becuase if he'd make this mistake once, it could happen again. She told him how heartbroken she was...