So, if he dates a girl who's bi but isn't attracted to you, or you're not attracted to her, will he dump her because it doesn't match his fantasy? What if you two *are* into each other and end up forming a romantic relationship and she find she's no longer feeling it with him -- could he handle that?
You guys have fallen for the unicorn hunter fantasy twice, the idea that you can bring a woman into your existing relationship and share her and it will magically work out. As they say, fool me once, shame on you, fool me twice, shame on me. Do you really want to get fooled three times?
You've wised up to the fact that life and love are complicated and that you can't plan in advance how things will go. Why can't he? I agee with Derby that it sounds like a control issue. If you're sharing *his* partner than you can technically have another lover without him having to actually open up to the scariness of you having an outside involvement. He needs to work on letting go of this, rather than clinging to fantasies that have failed you multiple times.
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.