I've been in mono relationships, like most poly people. Polyamory is a lifestyle, IMO, not an inborn sexuality. I think humans probably naturally lean towards monogamy for child rearing, but naturally seek out more than one partner otherwise. I saw a special on TV that says women prefer more masculine men during ovulation, and more feminine men when not ovulating, and I suspect that's true - We prefer different partners at different times.
I've always been kind of transient in my sexuality. I have periods where I don't even masturbate. I have periods where I feel very lesbian. I have periods where I feel very straight, and then I feel very interested in MFF threesomes or having sexual feelings towards men and women. I have periods where I think dating an MTF would be amazing, post-op or pre-op. Trans partners have this appreciation for being themselves, and seeing them become comfortable with their true spirits and identities is not just liberating for them, but their loved ones. Some of the best sex I had was with a trans partner.
I've also read women have more adaptable sexuality. I agree; I tend to find I'm interested in who I'm around. If I'm dating a woman, I tend to have more interest in women, and vice versa. And I imagine the same may even be true for poly - If you're actively living a poly life, it probably seems better than mono.
OTOH, I'm as vanilla as they come, and I cannot get into kink. Being dominated is not cool at all for me. I can do a good job at domination (so I am told), but it does nothing for me. I'd do some mild domination for someone I love, but they can forget me being a submissive. They'd have to find another partner.
I think it's a little sad your crush's OSO is probably going to end a relationship over a partner being too vanilla. Sex is important, but ending a relationship because a partner doesn't like a certain sexual act is unfortunate, but I guess I'm biased being a vanilla.