My boyfriend and I joked about fishing for Kaizi. He said we should break up. . Did he go too far or did I go too far?

You are going too far.

Because you were the first to say it. For example, if you stab him and he stabs you again, which one is more egregious? The one who strikes first is definitely too much. If the things you two are doing are equally excessive, the one who did it first must be more excessive, because he did it because you did it, and you were just free.