<human>: I don't know how explain this. I don't feel guilty because I didn't have an obligation towards this person but I also don't want their friendship to be ruined or strained because of what happened. 

We've hooked up a few times and he caught feelings. I was honest and said I had them too but I wasn't interested in dating anyone and that we were friends first. And I thought he was okay with that. Whatever I felt fizzled out and I started flirting with his best friend. His friend asked him if it would be okay if we had sex and he told us to do what we want. I'm not interested in playing games or trying to analyze his response so I did what I wanted.

Now he's pretty upset and won't really talk to either of us. He's more upset with his friend. How should I go about diffusing this situation? If I thought he'd care this much I wouldn't have done it. But we're all young and should do what we want and I don't want their friendship ruined over something physical. I just don't know what to say to make him get over it.

Can you give me a shortened version of the text?
<bot>: Slept with my former FWBs best friend, he's upset even though he told us to go for it, their friendship is rocky now, what do