Just to chime in: it's entirely possible and not at all contradictory to believe in human rights and other base ideals/"moral axioms" while being a secular materialist. The only difference is in where the belief is justified.
I consider myself a secular materialist in the way it's been talked about here, yet I still hold those views that you deem "idealistic". I just rationalize their origin in a different way.
I always cared about those ideals (human rights and such) since I can remember, yet I hadn't even heard of the concept of God before the age of eleven. I vividly remember thinking "Damn, they gave the creator of this Universe the same name as the word they use when venting in frustration!"
Right, but I'd argue that if you believe that (for example) human rights are some kind of fundamental fact of the universe then you really aren't that different from someone who believes in God.
If you just have a preference for a world where people behave as though human rights exist, then you can be a materialist.
> believe in human rights and other base ideals/"moral axioms" while being a secular materialist. The only difference is in where the belief is justified.
Anything beyond convenience and conflict resolution is justified by a metaphysical belief in human rights.