My boyfriend slept with someone else but he said I told him to, should I be upset?

Anonymous
Yes it's true, 2 weeks ago I told him ''maybe it's better if we just have a casual, open relationship'' but it was during an argument.
I was sick of tired of him giving me ultimatums and then saying that if I make another mistake, he'll cheat by mistake. It was then I said all that and was like ''I'm going to register on a dating site today''. I admit, yes I did registered and got 10 replies so fast. I went out on a date but nothing happened.

But then he goes to sleep with another girl. Yes it's not cheating but can't help it... I'm still pissed. I don't just hop into bed too fast with strangers. Talking to guys online, flirting with them and going out on a date (wanting to know them first) is different than actually sleeping with someone so fast.
Updates
+1 y
I have to admit it... part of me expected him to fight for me or act somewhat jealous. He didn't do any of that.
My boyfriend slept with someone else but he said I told him to, should I be upset?
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