I [25F] found out my boyfriend [26M] of 1.5 years is roleplaying on Reddit and lied about it

So the story is this: I asked to use my boyfriend's phone to look up how to remove gel nail polish a few nights ago. My phone was in the other room and I didn't think anything of using his and he handed it over. I looked up my stuff and x-ed out of my tab, and noticed he had Reddit up (on an inbox/messages page). He'd previously told me he didn't use Reddit and didn't know much about it - he said he'd used it in the past but not anymore. So I was a little suspicious... why lie about that? I asked him what he was messaging people about and he told me it was nerdy stuff (he's really into DnD & board games/etc.). I could tell he was lying and after giving him some cold stares, he finally told me that he liked to write stories (erotica) about his kinks with people.

I didn't know exactly what that meant, so I asked to look at the messages. Yeah... his posts went something like "I'm an 'age/sex/location/sexual orientation' into 'long list of kinks' and want to play!" He was sending and receiving a lot of sexually themed messages back and forth to people. Sometimes he gave them a link to Chatzy, which I'd never heard of.

I was pretty upset. Backstory - I know my boyfriend is kinky. He's told me some of the stuff he's into, and we've integrated some of it into our sex life. I've always had a suspicion that he wasn't telling me everything he was into because he'd evade my questions about, for example, what kind of porn he's into. I watch porn, I read erotica, I have my own less-than-wholesome kinks. I honestly don't care if he's the same way. I knew he masturbated and looked at porn and I know that's a typical part of a relationship. He and I have also been through a lot together... like, personal tragedies, sudden family deaths, run-ins with the justice system, and so on in our time together. We've both commented on how close we've gotten as a result of the emotional turmoil in our lives. He's repeatedly told me that he's been cheated on before and trust/honesty is the most important thing to him. He says he wants to feel that I "belong" to him - not in an abusive, possessive way, but in the body/soul kind of sense, that we are there for each other. As for me, I've drunkenly made out with a couple guys before (and told him afterward), but I've been totally faithful and apologetic afterward, and I quit drinking almost a year ago and haven't done anything like that since. I liked our relationship a lot, I was really in love with him, and I thought I gave him ample opportunity to tell me about his kinks without judgement.

He said that he was scared/embarrassed/ashamed to tell me about this Reddit stuff because of the content... that I'd think he was a freak. The content is some weird stuff, some of it gets a little gay, some of it's about transformation porn (like, guys turning into girls), some is about pregnancy play, and so on. Yeah, it's not anything I have experience with, but the content isn't the point. The lying and secrecy is.

And, what's more, is that to me it seems very much like he is not just "writing stories." He doesn't use that term in his messages, as far as I can tell. He seems to be roleplaying sexual fantasies out with random people on the internet. In my eyes, he's using other people to get off. He's having back and forth, rapidly paced conversations and basically cyber-sexing them. He's playing as a character and another person plays as a character, in first-person, and exchanging sexual messages. I wanted him to just own that that's what he was doing, and it took me three days for him to admit it was anything more than merely collaboratively writing stories. What he repeatedly said was that he used the terms "play" and "roleplay" as shorthand, because no one actually says "let's write a story." But, he finally admitted that he'd let things sometimes "get out of hand" and become roleplay, back-and-forth messages with someone. I keep asking him whether that's a core part of the kink for him - not merely reading what others have written and contributing parts of the story, but basically living out the story with another person. He says he tries not to think about the other person and doesn't care about them and doesn't want to get to know them. But, it seems like a lot of BS.

So I don't know what to do. I feel like an idiot, but I know there's nothing more I could have done to find out or get him to open up to me. At the core of it, I know that if he really needs to bring other people in for a sexual outlet, then this relationship isn't going to work. Again, I don't mind if he's looking at porn or whatever, but when it becomes a more personal, intimate experience with someone, even if he's playing through a character, I have a problem. Especially because he didn't seem to have plans to tell me.

I've received varying advice from people - some say that if it had been physical, in-person cheating, it would be way worse. That guys can objectify something and get off on it and be done with it. But my trust feels so violated. I don't know where to go from here. I still feel like there's more I want him to own - maybe that he liked the talking to other people, and that he chose to let things "get out of control" and become a conversation where he was personally involved in talking to someone else to get off.

tl;dr: Found out my boyfriend is using Reddit to do what seems a lot like roleplaying with other people online. I'm really hurt. Not sure what to do next with the relationship.