Great question! I was pondering something similar, but will post a separate thread about it...
My first response is that I don't think poly is about "tit for tat", so to speak. Well, you have someone else, why can't I?
Of course, if a relationship has an indefinite arrangement of partner A is allowed to date others but not partner B, then I think there's something deeper going on that needs attention. (I'm thinking of a thread I read a couple days ago about this).
Relationships are organic and need time/space to grow and/or change. I think it's understandable and reasonable for all "limbs" of a relationship(s) to ask for time to adjust to a new member.
And it's natural/understandable to feel jealousy...
my two cents