There's a large community where you are expected to be open to anything except sex with children; and a large community where you are expected to not be open to anything except sex between a monogomous man and woman.

I'm not arguing whether either of these points of view is valid. But both have enough adherents that no position that can be characterized entirely as more liberal or less liberal can identify its holder as rational. Therefore, anyone who says that such a position (for instance, being open to polyamory) indicates rationality, is merely stating their tribal affiliation. The fact that they think that such a stance demonstrates rationality in fact demonstrates their irrationality.

I can think of a few possible exceptions (sexual practices that are far enough beyond the pale that even tongue-pierced goths disclaim them, yet which have no rational basis for being banned), but they're too toxic for me to mention.