Loyalty during dating?

I meet a guy back in march online. We fell off but within the last month we started dating and seeing each other in person. It was going amazing and I wasn't seeing anyone else. We're not in a relationship but I've never been able to date multiple people. It's not my thing. Well I found out he was still on the dating app and had meet up with another girl during the same week that we had our first date. He told me all of this and said he wouldn't even be seeing her again. He deleted his account today. But he's doesn't really see why I was upset about it. I wanna know if he right and I shouldn't be upset or is this a red flag and I shouldn't trust him?
Loyalty during dating?
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