I "cheated" but we weren't dating.. How do I fix it?

so one of my long time friends and I have recently been testing the waters to see if we can be more than friends. Over the course of our friendship we've made-out a few times but that was the extent of it. For the last month and a half we've been spending a LOT more time together 1 on 1 and have slept together twice and having sleep overs once a week. That being said there were never any rules established nor did we talk about how we felt about each other.

The other night we were out with a group of friends. I wanted to go home alone but he kept insisting he wanted to come with me even if that meant just sleeping next to me. I felt overwhelmed because i felt obligated to leave with him and declined, and instead went home with a mutual friend who I used to be intimate with. By morning he found out we had left together and deleted me on all social media and told me he regrets wasting time on me and that he doesn't want an apology he just wants me out of his life because i slept with his friend again.

Now, like i said there was no exclusivity established at all nor had we talked about even liking each other. i thought it was very casual, and was and still am interested in seeing where it goes but at a slower pace. Yet his actions seemed like those of someone who's significant other of years had just cheated on them. Now i'm wondering what you guys think. Was his reaction warranted? How do i go about talking to him and trying to at least maintain our friendship.
I "cheated" but we weren't dating.. How do I fix it?
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