Personally, I think your wife deserves to know who you really are and how you really work. I think you will just end up feeling more isolated if this is a part of yourself you feel you need to hide. If you two really do trust each other 100% then she'll understand that you're not telling her in an attempt to be manipulative and that you mean it when you say you'll respect her boundaries. It may not be what she wants to hear, but we owe our partners our true selves. If you do decide not to say anything now then I would at least plan to tell her at some point, after you've settled a bit into life with the new baby.
Mags is right -- your sister wife scenario is ideal for you but it's not fair to think that it would make sense for Z in the same way. If she had a true friendship, trust, and love (romantic or platonic) for A too, that would be one thing, but it takes a long time to get there.
A vee isn't cheating if everyone is open and honest about it and it's based on a context of mutual respect and communication. It can make a lot more sense for a lot of people, since it's rare that two folks feel the same way about one other person. It can also build a base for a more triad-like relationship in the future.
And yes, hoping that she could be cool with you loving another woman when you wouldn't be able to let her love another man is not ok and would not make me feel amenable to sharing if I were her. It makes it seem like you want whatever you think is hot/fun but aren't willing to give what you hope to receive. Is Z even bi?
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.