what's wrong with a promotion of polyamory?

Maybe an exploration of the notion would be a better way to go about it. Many/most people have negative reactions to polyamory, and thus an article that takes a default view that polyamory is something to work toward will garner more negative reaction than you intended/desired.

Finally, the entirety of The Sequences proclaims "if you do this right, you can be just like me (Eliezer, trained rationalist)." Were you similarly made uncomfortable by that aspect of The Sequences?

Better when it's framed as "if we work on this, we can do better," not "if you do this, you can be just like me." Some community members see this place as mutual-improvement, not aspire-to-be-Eliezer (or Luke, or Anna, etc.).