Identity politics, moral realism and moral relativism Constructivism claims that all assertions of supposed facts are in actuality just social constructs, ways of thinking about things put forth merely in an attempt to shape the behavior of other people to some end, in effect reducing all purportedly factual claims to normative ones. That is to say, in claiming that all of reality is merely a social construct, such constructivism reframes every apparent attempt to describe reality as actually an attempt to change how people behave, which is the function of normative claims.
On such a view, no apparent assertion of fact is value-neutral: in asserting that something or another is real or factual, you are always advancing some agenda or another, and the morality of one agenda or another can thus serve as reason to accept or reject the reality of claims that would further or hinder them. This is simply the flip side of the same conflation of "is" and "ought" committed by scientism: where scientism pretends that a prescriptive claim can be supported by a descriptive claim, constructivism pretends that all descriptive claims have prescriptive implications.
Constructivism responds to attempts to treat factual questions as completely separate from normative questions (as they are) by demanding absolute proof from the ground up that anything at all is universally factual, or real, and not just a normative claim in disguise or else baseless mere opinion. So it ends up falling to justificationism about factual questions, while failing to acknowledge that normative questions are equally vulnerable to that line of attack. Thus such constructivism is tantamount to cynicism with regards to factual questions, inevitably leading to ontological relativism.
An objection to relativism is thus a reason to object to constructivism.
In terms of moral relativism, there are three different senses of the term "relativism" discussed in the field of ethics:
-One of those three senses, called "descriptive relativism", is merely the view that there are in fact disagreements about what is or isn't moral. I am not against that view, and I agree that there are in fact disagreements, quite obviously.
-Another sense, called "metaethical relativism", is the view that in such disagreements, nobody can possibly be any more or less correct than anybody else, that there is no way of resolving such disagreements. That is the kind of view I am against, in that it claims that there simply are not universally correct answers to moral questions, only different opinions, none better or worse than any others.
-The third sense, called "normative relativism", holds that because nobody can possibly be any more or less correct than anybody else, we morally ought to tolerate differences of moral opinion. While as already stated I disagree with the premise that nobody can be any more or less correct, I am nevertheless broadly sympathetic to the view that we ought to be rather tolerant of disagreement anyway.
Though philosophers do not usually give them names, I think we could usefully distinguish between a similar three different senses of ontological relativism, or relativism about what is real.
-One of those senses would hold only that there do in fact exist differences of opinion about what is real; and with that I would agree, just as with descriptive moral relativism.
-Another sense would hold that in such disagreements, nobody is any more right or wrong than anybody else; and with that I would disagree, just as with metaethical moral relativism.
-A third sense would hold that because nobody is right or wrong, we ought to be tolerant of disagreements; and like with normative moral relativism, I would disagree with the premise of that, but largely agree with the conclusion: though it's possible that in disagreements about reality, someone is right and everyone else is wrong, we should generally be tolerant of such differences of opinion.
The reason to be thus tolerant about differences of opinion, whether those are factual or moral opinions, is precisely because to do otherwise would lead to relativism. Universalism (of which the moral realism you contest is a species) logically demands freedom of opinion, unless it's also going to completely abandon criticism and take refuge in some kind of dogmatism. Unless someone's word could make it so, which it can't, to claim that something is universally right entails that any claim about what specifically that is might be incorrect, precisely because there is more to the claim than just an expression of subjective opinion.
I suspect it's really the dogmatism you're against, and you think that relativism is the only alternative to it, but it's not. You can be both universalist and also critical. That's a no-brainer nowadays when it comes to claims about reality: all of the natural sciences are founded on a critical univeralist approach. It's kind of exasperating that so few people can even contemplate the possibility of it when it comes to claims about morality.