People do come up with reasons for Jewish Laws that seem silly to us. eg. someone might say that the mix of materials in clothing is symbolic of God not wanting Jews to have syncretic religion.
But whatever reason one comes up with the fact remains that for Jews the mix of materials is against God's Law for them.
For you it is not a law and is not morally wrong.
But your argument also could be leveled against scientific claims like "The Earth is round."
A flat Earther can come up with reasons that the Earth is not round. Some of them like to point out that "the horizon is always at eye level." I don't quite understand what that means. But still, that counts as a justification for the belief that the Earth is flat for them.
For me, that does NOT count as a good reason to believe the Earth is flat.
My overarching point here is that just because opinions are different, doesn't mean that some views don't capture reality more precisely than others. Even scientists may disagree about the exact shape of the Earth. Just because there is confusion on the matter, and not everyone agrees, that doesn't mean there isn't an objective truth about the shape of the Earth "out there" that we can discover.
Am I convinced that my determinations about morality are correct? Ultimately: no.
I could be
wrong about stealing. Maybe there is no objective morality at all. But the fact that other people disagree with my moral views cannot be what makes them wrong. The only thing that can make my moral views wrong is if I have misjudged the truth of the moral claims to which I subscribe.
ie. Flat Earthers aren't wrong because they disagree with scientists. They are wrong because the Earth isn't really the shape that they think it is.
How is morality objectively decided by an atheist on the spur of the moment when a decision has to be made?
The answer might be that they do what feels best at the time and either condemn or justify themselves later in the light of day.
I think doing the best you can is the best we can ever hope for. It's the same with math exams. When you are solving math problems during a math exam, you might get some wrong. Does that mean you shouldn't try to figure out the correct answer? Does it mean that there IS no objectively right answer just because you get the answer wrong sometimes?
I don't think morality is something that "can be decided." It's something that we have to figure out.
There are three main metaethical theories that contemporary ethicists work with:
1) Moral Realism
2) Relativism
3) Moral antirealism (sometimes called moral nihilism).
To me, the real question is between 1 and 3. (Moral realism vs moral antirealism). Moral realists (for various reasons) think that some moral claims can be true. Antirealists (for various reasons) think that no moral claim can be true.
You could think of antirealists as "atheists about morality." They think that morality is a fairy tale... some human invention. Much like Odin or Yahweh. Just some made up sh**. I respect that view. Even if I express disagreement with it.
Moral relativists think that it boils down to individual or cultural opinion whether a moral claim is true or not. To me, that's just absurd. It's like saying the shape of the earth depends on whether you're a flat earther or a contemporary scientist.
Either morality is real or it is not real. People's opinions and beliefs, including my own, do not determine the truth of the matter.
Relativism is like saying that Zeus DOES exist according to the ancient Greek opinion. But according to the Ancient Egyptians, Zeus does not exist.
Who cares? Either Zeus exists or he doesn't. Even people who don't believe Zeus exists acknowledge that, in the view of the average ancient Greek person, Zeus DID exist. Likewise, moral realists AND antirealists acknowledge different opinions about morality exist.
Lets consider some cannibalistic tribe for a moment. According to the established folkways of that particular tribe, cannibalism is fine and perfectly permissible. Even a moral realist (whether she thinks cannibalism is wrong or not) can recognize that
according to the dictates of the cannablistic tribe cannibalism is good and permissible. Even a moral antirealist (who thinks that all moral questions are founded upon misunderstanding) can recognize that
according to the dictates of the cannibalistic tribe cannibalism is good and permissible. Relativism doesn't dare to claim anything one way or the other whether morality is real or not. So I think relativism is pretty useless as a theory. It explains nothing that competing theories can't explain and relativism doesn't even TRY to explain anything more than them.
But the other two theories (realism and antirealism) acknowledge everything that relativism does (ie. folks disagree) and then BOTH of those theories go one step further and try to determine what is actually true. That's why I reject moral relativism. For its uselessness. Not just its falsity.