So I'm often reading here about setting the boundaries in a relationship (telling her what is unacceptable). I've also read the 6 stages of branch swinging.

My girlfriend messaged me and asked "can I have a dinner with Chad tonight"? I said "nope", she said "ok", and that was the end of the conversation. We then talked about unrelated stuff.

The Chad is actually an extremely boring beta, but one with rich parents, unlike me. She clearly understands that he is interested in dating her.

I'm planning to talk to her next time I see her and set the boundary. Can you comment on whether it would be dumb or how it can be improved? Does it cross into mate guarding?

"Hey, I want to be clear with you that it's unacceptable to meet men who are clearly into you. I won't tolerate you finding backup options while we are together."

If that matters, I'm 30 and she's 23.