Very well. In that case, I continue to express my disapproval for a variety of reasons, including:

1. If someone really does succumb to influences that lead away from their preferred outcomes due to emotional insecurities, then acting derisive is likely to only make them defensive and less likely to actually change their behavior.
2. In general, I think we should seek to foster a positive atmosphere on LW. It makes the site more pleasant to read, and avoids the risk of misunderstandings (like mine above) that arise from provocative language.
3. Even though your hypothesis of "this is emotionally suboptimal behavior for a large fraction of people" does sound plausible, there's still a considerable chance that you might be wrong. It seems to me that a good heuristic is "be very careful about the kinds of thinking you're loading with negative affect, or people might become unwilling to properly update in favor of that thinking afterwards".
4. This is especially so since regardless of whether or not that's right for the majority, there are almost certainly bound to be people for whom that kind of behavior is in fact the emotionally optimal one. Loading negative affect on that behavior can cause them lasting emotional harm.
5. Expressing things in strong and confrontational tones will also make it socially harder for you yourself to update in a different direction afterwards (if that turns out to be necessary), so that sort of behavior should be discouraged.

More precisely I would say that I object to the act of succumbing to influences that lead one away from preferred outcomes in an obvious way due to maladaptive emotional insecurities. That happens to be a flaw in human psychology, a maladaptive left-over from a different environment that has particularly negative consequences, is often exploited at a cultural level and that I have a personal vendetta against.

I agree with you on that, but I don't see why you think that's what's going on with consensual polyamorous relationships.