Found out that my fiancé (20M) cheated early into the relationship, how do I (20NB) bring this up with him?

Before we start, I’m not looking to get the “you’re too young to be getting married” or “this is what you get for trying to get married too early” comments. I never imagined myself getting married until I met my fiancé. My fiancé and I have discussed in detail about what we want in our individual lives and they align greatly. We compromise easily without giving up our own values because our values are pretty much the same. We genuinely can’t see each other with anyone else. Because of all of this, we figured marriage makes the most sense. We already have the marriage license.

I’m also not looking to hear the “well what did you expect from someone who just wanted to fuck”. I want to believe that this relationship has grown to something more than just sex. We have gone through difficult financial, emotional, mental problems and we’ve supported each other every step of the way. Is this not what happens in other long-lasting relationships?

I (20NB) have been dating my fiancé (20M) for 221 days (this number is significant later). We met on reddit after I made a nsfw post looking for fwbs. He messaged me first and our conversations weren’t that sexual in nature although we did briefly discuss sexual preferences. He was still genuinely interesting and we agreed to meet up later that week. We ended up liking each other’s company more than we realized. He asked me out and I said yes.

We have always been there for each other. If anything, I feel like he’s provided more esp financially since he works. He always tries to make me happy and whenever I have an issue that I want to discuss with him, he’s mature and empathetic. We’ve met each other’s family and are accepted/welcomed which is decently important to us.

Now back to today. I get anxious easily and my ex cheated on me by looking for someone on reddit and lied to me after I caught him saying that it was a coincidence that someone else with his name, age, description, major, and interests happened to be looking for a hookup around the same time that we were going through a rough patch. Because of this, I decided to check my fiance’s user (the one he messaged me with) and I saw he commented on a nsfw post where someone wanted a creampie. He commented that he was “down” 218 says ago. This means that three days after we had started dating, he decided to try to find someone else to fuck.

I haven’t had the slightest reason to feel like he’s interested in anyone but me. He always tells me how much he misses me when we’re not together, he has driven to me in the middle of the night when I’m anxious when I’d be staying with my parents, he shamelessly commits pda, he loves showing me off, etc.

Idk what approach to use to discuss this with him. My only defense for him is that since we met on reddit with the expectation of finding a fwb, he didn’t know how serious to take the relationship when we started dating. Esp since he’s been cheated on before, maybe he didn’t want to let his guard down? I don’t know. It hurt to find out though because our wedding was supposed to be this halloween.

Ideally, I want this to work out. I want to have a serious, mature conversation with him. At the same time, my trust has been broken and so anything he says from now on gives me feelings of doubt and I fear when I try talking to him I would only lash out with yelling and tears.

TL;DR: My fiancé wanted to hookup with someone else 3 days into our relationship and never told me. How do I discuss this maturely? What should be my next course of action?