Sounds like S wants to know that J (or any new person) has crossed from "friendship" into "dating potential" in your mind before you talk to the new person about changing the state of that relationship.
Is she worried about being blindsided or railroaded into something that she feels she needs time to prepare for?
Sounds like you both had different ideas of where that "I need to speak to my partner first" boundary really was, and I'm sorry this turned into an argument rather than a discussion point to figure out where that line REALLY exists for each of you (and determine a compromise that works for the both of you). Hopefully, you and S can talk about it with clearer heads and try to iron out the details now that you know it wasn't completely clear.
Mono. Divorced, two kids (DanceGirl, 14; and PokéGirl, 11), two cats, one house, many projects.
My partner. Poly. In relationships with me, Xena, and Noa.
Poly. In a relationship with Chops. Dating others.
Married, Poly. In a relationship with Chops.
Blog thread: A Mono's Journey Into Poly-Land (or, "Aw hell, there's no road map?!")
Slightly more polished blog with a mono/poly focus: From Baltic to Boardwalk