Here’s a thought experiment.
Let’s say there’s only one person left on the planet. Kinda like that guy in one of the Twilight Zone episodes where he was the only living thing left on the planet so that he had all the time in the world to read. So this one guy is left on the planet – can we list some things that he can do that are immoral?
Of course not.
Morality is all about how we interact with out fellow human beings. Something is “immoral” only if it causes harm to another sentient being, harm being defined by the wronged being. Basically, there has to be an affected “other” in order for morality or immorality to exist. If there was only one person on the planet, then everything is amoral: morality doesn’t exist.
Keep that in mind. Morality can only exist if sentient beings that can be wronged exists.
Now what about objectivity? Here are some definitions:
1. existing independent of thought or an observer as part of reality.
2. undistorted by emotion or personal bias; based on observable phenomena;
So the key component to “objectivity” would be something that exists independent of thought, independent of an observer, independent of emotion and of personal bias. So, grass being green is objective. While the color that we have in our heads when we think of “green” might not be objective (since it’s subject to our perception), grass will always absorb every color of the wavelength except for the wavelength that we’ve deemed “green” – even with no humans around to perceive it and call it “green”.
Now, let’s put these two words together: objective and morality.
This would be a system of morality that’s independent of biases and emotions (objectivity), yet dependent on biases and emotions (morality)! Morality can only exist if there are beings who have biases and emotions, yet objectivity can only exist if all biases and emotions are removed. Morality is subject to any sentient beings’ biases and emotions.
The two can’t possibly coexist; meaning that the phrase “objective morality” is about as meaningful as a square circle.
Some believers might invoke “god” at this point, but if god has emotions and biases then it’s not objectivity. Unless the god being invoked is a non-personal, pantheistic god, it also brings up the immorality of the Sovereign Defense. If “objective morality” is being dogmatically assigned to a being with agency (like the Christian god), then immoral acts are bound to happen.