I absolutely agree with you, sagency, re: no ultimatums. I don't like them and honestly I feel like we can work through nearly anything if we all try.
but I do think that plays a part, maybe more so than if I were a man. I mean, I think she understands my desire for a male partner (although in reality I don't want a male partner, I just want Z), but she may not be comfortable enough with the idea that Z wants another woman.
We had an issue around the same time that Z and I established our actual relationship in which a (female---Z is straight) poly friend of ours asked for permission to ask Z out. Q gave her permission, as did I, for them to go on a date, but after some thinking I realized I wasn't too thrilled about it, and asked them to hold off on pursuing it further until things with Z and I were more established and comfortable. Q told me then that she was happy to have it just remain the three of us. She did say at another point that she would be fine with me dating other people, but again, she has a track record of thinking she would be fine with something and then not actually being okay with it. And I'm not interested in dating other people, honestly, casually or otherwise.
I'm not entirely sure what, exactly, it is that bothers her, because see also: she won't talk to me. I'm sure she doesn't like feeling left out when he and I spend time alone together---I don't like feeling left out when they spend time alone together either! But she's also made it clear that she doesn't want us having sex, and she doesn't like that he and I often do end up snuggled in bed together when we're alone. I wonder if she also feels a little like she was lied to, since both of us made it clear that we weren't interested in having another emotional relationship, and yet here we are in one.
Actually, I think what I need to do at this particular moment is stop trying to suss out her feelings and motivations and get some sleep! Heh. I might try to reach out later in the week when this is less raw and hope that she's willing to try to find some answers with me. We'll see.