Derek thought things would get better after the trial but they only got worse. For him at least.
His stupid idea wouldn't leave him alone, especially because Joy found a job working as a teacher's aide at a preschool and talked about how much fun she was having with those kids all the time. She had been telling the truth before about being okay around kids again once she knew Rachel was okay. He hadn't been so sure at the time.
He was glad she was happy but it also sucked because it made it easier for him to justify things. It wasn't too late for her to have more kids of her own and not have to miss out on their development this time.
Derek wasn't the best candidate for that but he was the only one currently willing. And he couldn't say anything because it would definitely come out sounding wrong.
Joy didn't think of him that way. Never had. Never would.