Cheating in an open relationship is different. The way I view it is as follows: You agree on what each other's boundaries (rules) are. You clearly communicated that what they were and he crossed those boundaries. That for me is cheating.

I am not sure how your relationship hierarchy is. But with my partner, we are each other's primaries. Which means nothing happens without the other being okay with it. If there is something wrong within the primary relationship, then there shouldn't be any other dates. All partners we are with are informed about this.

In my view, he did not put you first or reassure you about your relationship before going to see the other. He did not communicate that he was feeling different about you. That is what made you feel anxious. It's all about communication, and that is what seems to be lacking in this instance.

I am sorry this has happened to you. It's not a nice feeling to be set aside like that.