Welcome! I love your story and I get what you're saying being afraid that J will want more since he could assume that being poly means you're "available". Just remember that being poly does NOT mean you have an obligation to be with just anyone who wants a piece of you! If you don't want to date him, whatever you're reasons, that's your right!
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack