Yes, Opalescent nailed it. By referring to the social default, I was indeed referring to the surprise that people (mono or otherwise) typically express upon learning that someone (mono or otherwise) is still friends with an ex partner or lover. Even though we all know people who have positive "afterships."
(LOL, NYCindie, I understand you hate that word. I just don't think that "friendship" adequately describes the range of post love/partner relationships I've seen. And here, what I'm discussing refers to a specific type of connection dynamic. It's got its own issues, which are the subject of my post. YMMV :-)
I'm not saying polyamory is more "evolved" -- but as Opalescent notes, anytime you step out of the mainstream, questioning or ceasing to follow the standard social script, that's when you seriously start to learn more about relationships.
I'm friends with some exes, not with others. The pattern I've noticed in my own post-lover/partner relationships (and those of many other people I've discussed this with) is that where we've each acknowledged to the other how we behaved, what our true priorities were, and why, that's a big part of a strong foundation for continuing friendship -- or at least a truly amicable parting of the ways.