Pain is the body's way of telling us that something is wrong.

Interesting phrase here. "Wrong" here cannot be understood as a moral category, right? If I punch you, on the basis of my subjective morality, there's no pain in my body. Why should I care about the pain in your body? To what can you appeal here?

You can make a case for an objective standard of morality, and I can also keep asking you "why?"

Totally! And thankfully my answer isn't as silly as "just because! And if you don't, the only persuasive thing I've got in my arsenal is to lock you up!"

It was to explain why they shouldn't do what they're doing.

According to whom? To what are you appealing since morality is subjective and this subject thinks it's fine?

Try not strawmanning me.

Uh, drawing out the logical implications of what you say isn't a strawman, evne if you don't like where your own words lead you.