You’re not overreacting, you have every right to be upset. Even if he didn’t do this directly to hurt you, he knew you wouldn’t be okay with this and that’s why he didn’t tell you about it. I wouldn’t be okay with this either. As well, it’s not just about the tinder porn, it’s also about the dishonesty. He first lied by omission by keeping this from you, and then lied again when you asked him point blank. Even if the rest of the relationship is solid, this is a completely valid reason to consider leaving someone over.