As a bisexual woman, I've given much thought to whether I ought to date couples. There's a large swinger's community where I live. So if I chose to have threesomes, boy, would I be popular.
Ultimately, based on my own personal experiences in triads, plus things I read here and other places, I have decided I will not date couples. I might date both members of a couple, but not if they insist on being a "package deal." And I seriously doubt I'd ever agree to poly-fidelity with an already established couple. I want to choose my own partners to suit my specific life, my desires, my tastes and needs, rather than have someone chosen for me in an agreement that was made before I ever even arrived on the scene.
Sometimes, the male-female couple looking for their "third" comes off as insulting, as they seems to assume my (bi)sexuality might be put into the service of "enhancing" their heterosexual relationship. Yes, I find it annoying that so many bi woman seek other bi women strictly to "share" with their man. Like many quality women who like men, I usually have my OWN man around, and I would NEVER expect a woman I liked to have sex with him. To me, it cheapens the same-sex connection to insist on having someone's guy in the mix.
My actual experiences in long-term triads have taught me that I will always want one member of a couple more than I want the other. My longest triad (2 years) was me in love with the woman, and her boyfriend was my FWB. They weren't an already-established couple when I joined them (None of my triads have gone down that way, actually.) I was dating him first, then she came along, and we both fell for her.
She loved me, but she was in love with him, and believe me, I felt the difference. He and I weren't in love. But I felt that if I stopped sleeping with him, I'd have to spend less time with her. (We all slept in the same bed every night, her in the middle.) Eventually, I got tired of threesomes when I didn't really want the guy around, and tired of feeling like a third wheel, and I left the relationship. They're married now.
OP, there's nothing wrong with wanting what you want, how you want it, but maybe this helps you understand some of the resistance you're meeting, from the perspective of a potential "unicorn."