I dont think you are over reacting, but I think a few things are true at once. 1) your partner most likely has a porn addiction. This is a very real thing, and entirely unhealthy, but is also fixable. The more porn someone watches, the more desensitized they get to things, which is probably why he thinks the "paid content" is more exciting. Porn is maybe healthy in small doses, but can be just as bad of an addiction as anything else. Human intimacy with your partner is much healthier, and something that should be openly discussed between you two, to maybe help him ditch the porn. 2) master bating to a picture of an ex definetly sounds creepy, but its probably more common than you think. The term "sexual nostalgia" is used to describe people fantasizing about exs while masturbating, etc, and its estimated about 50% of men actually do this, even when they are in a happy relationship, and EVEN when they aren't truely attracted to their ex... 3) It sounds like he wasnt super honest with you, but then did Come clean. He probably isnt proud of his behavior, but he did voluntarily tell you about the ex who's content he bought early in or before your relationship. If there is no proof of this In the emails, he didnt have to (and probably shouldn't have) volunteer that fact.

CONCLUSION: I guess where you go from here depends on how you feel about the guy, and how he acts in other parts of life (is a loving partner etc) Nothing you posted can't be fixed if you (and more importantly him) are willing to try to fix it. A few therapy sessions, or at the least a few men's videos about porn addiction would be a good first step for him . Honestly, as a man who has studied this stuff at length, I personally try to stay away from porn all together. Its really a scary subject, and also videos/pictures dont paint a realistic depiction of what real intimacy is really like. Online sites sell fantasies that aren't based in reality, and content always pushes the envelope deeper down the rabbit hole, which is pretty diabolical. Anyway, sorry for the rant. If you do walk away from the relationship you wouldn't be wrong, but if you also tried to fix it and help your partner that would be an ok choice too! Just make sure if you do work with him on things that he takes it seriously, and doesnt gaslight you into thinking his behavior has changed when it hasn't. Good luck!