@Thomas,
This discussion is too much in the theoretical. I think practical examples help.
I am claiming that there is a fundamental difference between scientific debates over scientific facts, and moral debates over moral facts. Look at the difference between the debate over evolution and the debate over abortion.
The debate over evolution is based on what is observable and testable. We have radio-isotope dating. We have fossils to study. We have geological. And now we have DNA evidence. In each case there are testable assertions-- people can go get fossils. People can do genetic experiments. There are people who deny the facts-- but it is in ways that are easy to debunk.
The debate over evolution is a debate about the facts.
In the debate over abortion, people on both sides agree on the basic facts. We agree that abortion is taking a life. There is no question that abortion does stop a beating heart. We all agree on the point of conception and the point of viability.
Even among people who completely agree on the testable facts, there is a deep disagreement about whether society should permit this practice. (For those who want to go there, this has nothing to do with religion. There are atheists who are strongly opposed abortion and devout "religionists" who are strongly pro-choice.)
The question of evolution can be solved by science (and ironically the only people who question evolution do so for religious reasons). This is because evolution has all of the traits of a scientific question -- it is a well-defined question that can be studied and tested.
Moral Questions like abortion are impossible to solve in any objective way. People who agree on the facts can be on opposite sides of the question-- and there is no way to resolve this other than a political battle yielding to society making an arbitrary decision.