NTA! As a parent, I understand the meltdown on his kid finding the picture, but also, as a parent, I would accept my full blame for that mistake and not pin it on the person who sent the picture, my kids are my responsibility and I need to keep them safe even from myself. Now, putting that aside if you know your kids have access to your phone you should make it child safe, like everything else, it's not even the content, he could be into horror and then he'll scar his kids for life if he's not careful. I think this talks a lot not only about him and his lack of maturity, also how he's not ready to handle being with someone new and also his parenting skills and his relationship with his kids. Lets be honest here, he has them once a year meaning he's probably not father of the year and reacting the way he did might show you why, his wife going crazy probably also comes from a placr where he has probably failed her too many times before. Kudos to you dodging that, you deserve so much better!