TIFU by opening up my relationship

I'm being vague on purpose so I'm sorry in advance

So recently my partner and I opened up our relationship, I told a friend about this thinking they're either gonna be a safe first run at this new part of my life or things would be awkward for a little bit but our friendship would be okay. We already talked openly about our relationships and sex lives, why not shoot my shot?

Well my friend has a partner so the two of them talked about it. My friend got back to me saying everything was good but there were some rules, that's fine I have rules too.

The rules: I could have sex with protection my friend couldn't have sex with me at all. I had to tell my partner everything that happened, when where and most of the details, and my friend told their partner to the degree they were comfortable with sharing. I could be touched with no clothes on and I was only able to touch on them if they had a layer of clothes on but could take their clothes off around me. No catching feeling.

We both followed the rules and turned the limits into fun teasing. We flirted back and forth a lot, we hung out, and sent photos and videos. We did everything we were allowed to do.

Well after almost a month I finally met my friend's partner and things were a little awkward for the first 30 minutes but that's nothing we didn't expect and then we got along really well. We became friends, swapped numbers and texted, and really just focused on our friendship.

After a while my friend's partner and I were alone talking so I decided to bring up that we're getting along really well considering the circumstance. I didn't out right say it trying not to make things too awkward but they agreed that we got along well and that things were fine. Nothing was definitively said though so I pushed it a little more and again reassured that everything was good.

Things were not good. My friend's partner didn't know the extent of things and after we had talked one on one they found all of the texts between my friend and I. I was made aware of this and after I got over the initial shock all I could do was apologize. They understood that I was under the impression that everyone was consenting and I thought I had done nothing wrong.

My friend's partner wanted to talk with me and I agreed. Whenever they wanted to talk for however long they wanted to talk for I was okay with it. I wanted to give all the power to them to handle this however they wanted to. They're the one that was just hurt after all.

I haven't gotten a call or text yet and that's fine by me. Maybe they changed their mind or they're too hurt to reach out, whatever their reasoning is I understand and sympathize.

I have talked to my friend, I had questions and felt I deserved to at least be heard. It's only when my friend talked to me that I found out their partner didn't know about ANYTHING. Not just the extent of what we were doing but nothing. My friend kept apologizing after every answer and admitted to lying about other things.

I feel so bad for doing this and hurting someone. I know I didn't personally hurt them but I was still a part of it. I want to apologize to them again and show them support but that's not my place. My partner and best friend keep reassuring me I didn't do anything wrong, I was lied to and used, and maybe me and my friend's partner talking one on one sparked all of this which is a good thing.

A part of me still wants to be friends with the cheater because we were friends for awhile and they were a good friend I confided in but I know that's wrong.

TL;DR My partner and I opened up our relationship and I tried to have a friend with benefits but my friend cheated on their partner with me for a month and I had no idea. Now I feel so bad I'm telling strangers on the internet.