How do I 23F fix a communication issue with my 36M FWB?

please tell me how I can salvage this me f23 and my fwb m36 just had an argument? discussion? idk im still emotional about it honestly but i had a guy i used to mess around with come over last night and we had sex. I didn’t think of it as something that would bother my fwb bc he has a BM and they were with him all week, i k ew she would make passes at him and that thought didn’t really bother me bc duhh fwb

this afternoon, when my fwb called and, for transparency, I told him me and this dude had sex because i feel like it’s the right thing to do in these kinds of relationships. He’s upset, more-so annoyed at the guy bc he wasn’t gentle and I need to physically recover (negative sexual experience) and he said he’s upset because he thought there was more there, he assumed I felt it and realized it too and he didn’t think to bring it up. Now, if I had known he wanted to date me I wouldnt be making this post bc I would be with him lol like if I had known he wanted to date me I would’ve jumped at the chance to. Hes an amazing guy and while I entertained the idea before this, I kept it within the fwb confines. I don’t want to lose him at all, I’ve already told the other guy to not talk to me again, that I’m not interested (partly because of my fwb, mostly because I don’t want to have to recover after being intimate with anyone).

I feel ashamed and embarrassed and various other bad emotions because I unknowingly hurt my him on such a level, would there be any way to salvage this so a relationship could be possible? he says his feelings on me don’t change, that we’re still friends that he still finds me attractive and everything but I don’t want to lose him in any sense of the word. How can I show him, besides what I’ve already done, that I’m not the kind of person that cheats, that I’m serious about the possibility of dating him but we can’t do that if he has these feelings and doesn’t worth through them fully. I don’t want him to resent me for this

for further context we’ve been friends for a year and fwb for another maybe 8-9 months, I haven’t dated anyone since 2018-2019 and I don’t sleep with anyone else but the two men I’ve brought up in the post nor do I have any desire to sleep with anyone else so I think he got the initial impression that we were exclusive because I don’t have sex with anyone else besides him and the rare occasion I’d sleep with the other guy…it’s just a whole mess and I want to fix it and be his girlfriend