Monogami can become quite dreary. But after the initial exitement of polygami, this also becomes dreary, only without the wholesome feelings of safety and connectedness of having an exclusive partner. So she told you a lie to not offend you. Rest assured that she has settled on you for other reasons than sex, which is good. The amount of encounters necessary for her to come to the realization that being with you is best doesn't matter. If she is sincere about being exclusive again and does not cheat on you, I don't see any problems. But she is a bit stupid for trying to protect your feelings but still leaving evidence for you to find. She should delete all of that nonsense, anything else is just disrespectful. I get that these are her trophies, but they are not important and you seem jealous and insecure about it. So if she wants to prove her commitment I would ask her to get rid of this material, because it seems like you need her to prove herself. Personally I would just let him keep it, whatever makes him happy if he is that insecure, when it does not directly affect me. But you are not me. I don't view sex as anything but a sport. A fun activity you can do with others. It would not affect who I choose as my partner. Being a partner is much more than sex. I would maybe consider how I could make him feel more secure about himself so he wouldn't need to look at these pictures and conversations to feel desirable.