Therefore, anyone who says that such a position (for instance, being open to polyamory) indicates rationality, is merely stating their tribal affiliation.

I wouldn't suppose that "being open to polyamory" per se indicates rationality. But I would consider someone rational who, having thought about the matter, and concluded on the basis of sound reasoning that there is no valid reason to condemn polyamory, decided to adopt that lifestyle even in the face of some cultural opposition.

And I would consider someone irrational who, having no sound reasoning behind that position, would act in such a way as to deny others the enjoyment of a non-straight-monogamous lifestyle.

Controversies involving third parties are a valid matter of debate, for instance, I'd concede that there is some grounds to ask whether gay couples should adopt. But to assert, without argument, an interest in what consenting adults do behind closed doors, and that doesn't cause anyone lasting harm, just because it concerns sex - that does strike me as irrational.