My view on this is the following:
When one agrees to a polyamorous relationship, they are agreeing to allow their partners to love other people. Love is an organic thing, so is lesser forms of romantic attachment and the minute you start letting others control how you form this kind of attachment, you're moving away from polyamory. Simply because it isn't about loving people the way someone else wants you to. Any potential relationship you had with J was/is between the two of you and so before you spoke to (or more like sought permission from) S, you absolutely needed to see how J felt first. S seems to want to practice a form of ethical non monogamy where she will have the ultimate say over who you date and when. She feels that as you're existing partner, she should know how you feel about someone before they do. What this means for other relationships is that nothing can develop or progress spontaneously. Anytime you wish a relationship to progress to a new stage, emotionally or practically, you'll be expected to seek her approval first and wait for her to be comfortable with it before it can go ahead. Now, she might have good reason for requiring this from you. Maybe you're flakey or she is accustomed to people treating her unethically but it might not work for you. Some people are fine with this relationship style but others want the freedom to let relationships develop organically. Personally, my only rule is that my partner should keep me informed of game changing situations as soon as they can. If they expect that something new in their life will affect how or when we see one another, then I think I'm entitled to know so I can decide if it works for me or not. So, I don't have to know about someone new until s/he is a fixture on his general schedule. We are comfortable with being poly though, and like to hear what the other has been up to, so we gossip. It isn't disclosing though, it's general chit chat.
Just because I was his partner first, it does not give me the right to have agency over who he interacts with and how he interacts with them. I trust that he wants to be with me and will continue to maintain our relationship as long as it's beneficial and healthy for us both. That trust lessens my need to control his other relationships.
Please forward any comments, complaints or suggestions about my various thoughts and opinions here