I think I'm a bit appalled that someone would suggest that a wife who's husband is getting flirty with other women on facebook has trust issues that stem from childhood.

Here's an alternate theory. Your guy is telling you that something is amiss, as it should be doing. Your sense of right and wrong is kicking in and you know deep down that this is wrong.

I have a feeling that the last person to reply has stepped out on his SO and is using his reply as a justification for his own misdeeds.

Humans are designed to be monogamous. I know your relationship was open before, but maybe its basic human nature that's kicking in and saying hey, protect what's yours! That is, there's nothing wrong with these feelings of mistrust.