Social justice, apropos of the name, is largely an exercise in the manipulation of cultural assumptions and categorical boundaries- especially the manipulation of taboos like body weight. We probably shouldn't expect the habits and standards of the social justice community to be well suited to factual discovery, if only because factual discovery is usually a poor way to convince whole cultures of things.

But the tricky thing about conversation in that style is that disagreement is rarely amicable. In a conversation where external realities are relevant, the 'winner' gets social respect and the 'loser' gets to learn things, so disagreement can be mutually beneficial happy event. But if external realities are not considered, debate becomes a zero-sum game of social influence. In that case, you start to see tactics pop up that might otherwise feel like 'bad faith.' For example, you win if the other person finds debate so unpleasant that they stop vocalizing their disagreement, leaving you free to make assertions unopposed. On a site like Less Wrong, this result is catastrophic- but if your focus is primarily on the spread of social influence, then it can be an acceptable cost (or outright free, if you're of the postmodernist persuasion).

My general sense is that this is a fairly distinctive quality of social justice communities, so your feeling of alienation may have as much to do with the social justice community as it does with the LW memeplex. A random conversation about fat acceptance with culturally modal people might be a great deal less stressful. But then again, you probably shouldn't trust somebody else on LW to say that.

(I upvoted Simplicio and Salviati, by the way.)