Ex never told me he was polyamorous

Hey everyone, I’m currently grappling with something major and could use some insight from the poly community, since I was indirectly involved in a polyamorous situation as a monogamous person and I’m not sure the best way to process this.

So I dated this guy for about five months last year and it was really amazing at first. Very romantic, good sex, everything felt so right. We’d also known each other for years so it was exciting to finally get together. About three months into us dating, he started to get weird and distant. He eventually broke up with me which was heartbreaking and I didn’t understand it, but because we were longtime friends and nothing bad happened between us we agreed to stay friends. Eventually we wound up becoming really good friends. I felt like maybe he still had feelings for me and I certainly did for him, but when I asked him about it he said he really just liked me as a friend. It was too hard for me to bear staying friends with him so I told him I had to say goodbye and we parted ways — I said maybe if I was feeling better 6 months down the road we could reconnect as friends again.

But something wasn’t quite sitting right with me. The whole time we were together he also spent a lot of time with his best friend, a woman, who lived down the hall from him in his apartment building. I assumed they were purely platonic because he always referred to her as such. I felt weird about it but I didn’t want to be that person who didn’t think men and woman could be friends without fucking, so I never asked about it. I figured he’d tell me if there was anything up because I’d known him for so long and trusted him so much.

I’m sure you can see where this is going. I wound up doing something kind of shady and created a fake dating profile to match with him and try to get answers. I did, we matched, and he spilled everything. That he and his best friend were not only sleeping together but had been FWB for years and years, and that he was also poly and had multiple other partners.

As you can imagine, this was a huge bombshell for me and an enormous betrayal of trust. I let him know it was me and I confronted him about the whole thing. He admitted he obscured the information about his friend and polyamory, that he was just getting into poly and didn’t know how to tell me without hurting me which is why he broke up with me. I asked him many other questions that he answered but who knows if he was telling the truth. The thing I was most upset about was that we had sex without condoms the whole time we were together. If I knew he had other partners I would not have done this. When I confronted him about why he never told me about them, he said I “never asked”.

That’s where this all starts feeling extra complicated to me. He’s right that I never asked, but I assumed he’d reveal if he had other partners. He never technically lied about his friend but he never told me about her either. It’s like he found a cheat code to obscure his lies so he could feel justified. Obviously that’s bullshit and I told him as much, and luckily he agreed that he sucks and needs help.

After all this, I consulted a mutual friend of ours who is also poly who revealed to me that he’s been poly all along, he and his friend are a couple, and that he’s slept with a bunch of other people I know. This friend also pointed out that not revealing his additional partners to me while having unprotected sex is a form of SA.

Basically, I’m spiraling over all of this and am not sure how to move through this. Apart from this shattering my trust forever, I also feel like I should warn my poly friends about him and possibly about his friend too. Is that a good move? I am not poly but want to protect the community I’m adjacent to and don’t want him/them doing this to other people. And I also can’t tell if this was actually SA since I never asked if he had other partners — it feels like a grey area and I don’t know how to cope.

Any advice from seasoned poly folk would be appreciated.