Your article subtitle refers to "both sides of the argument" (i.e. objectivism and subjectivism), and I'd like to suggest that there is at least a third possible position here.

When we consider an agreement—like an international peace treaty, or something smaller, like a marriage or divorce—the language of "objective" and "subjective" doesn't apply. Clearly these agreements aren't written into the fabric of the universe and don't exist to be "discovered," but neither can any single individual simply "imagine" these agreements into existence. Rather, agreements have the validity that we as a society choose to give to them. They are part of a shared reality that we socially construct. They can be challenged; we can reason and argue about them; we can un-decide them and re-decide them. They don't have an objective reality, but they tie to our communicated agreements about our beliefs, and no single person's belief can independently rewrite such an agreed-upon reality just by an act of imagination. If someone has a new insight about an existing agreement, they have to communicate it and convince others to revise the agreement, or it doesn't become "true."

In my view, ethics is socially constructed and often functions in the manner I've described above. Personal freedom plays a role. Social constraints also play a role. We are free, but we can't just make up any answer we like. We participate in a shared reality. The third option is something like "interdependence."