Was I wrong?

I’ve been talking to this guy for 4 days, it was initially suppose to be a one night stand but we connected and opened to each other more than we ever had to other people. He was talking to someone when we met and he was honest about that, and all i said tell me when/if you cut her off.

Yesterday I went to a party and my ex and I ended up kissing and I told the guy I’m talking to about that. He was pissed and called me disloyal and decided to cut me off, he now doesn’t believe I could be trusted in a relationship.

I told him that I liked him but we never established that we would only talk to each other + he was still talking to someone else from my knowledge. Then he let me know that he ended it with the girl and showed me proof.

Now I’m very confused cause I thought it was normal to date other people until both parties decided to be official or only date each other?

Pls help understand!