What is objective morality anyway? I don't know what that is. I believe that atheists have morals too.
Objective morality in this sense means that something is wrong whether you believe so or not or said in another way whether you exist or not.
Subjective morality means that it is from the point of view of a subject (a human, alien, you or me etc.) and that it changes depending on the subject, meaning that you might think killing someone is morally right, while I think it is wrong because we are different subjects.
If God exist and as claimed is the moral judge, he has decided or is the creator of what is right and wrong and if he says that killing is immoral then that is wrong regardless of what you think. You deciding to kill someone or "celebrate" the killing of someone is inheritably wrong because God says that it is and therefore you would basically go against the will of God.
This obviously goes the other way as well, so when God say that certain sexuality is wrong and should be punished by death, you are not doing anything immoral for following that statement, because God says that it is good.
People like me for instance, which don't believe objective morality exists, but that it is subjective, will make the claim that there is no moral judge like a God or anything else for that matter which are capable of making a such judgement call, but rather that our morality is based on what society or individuals believe is morally right or wrong, based on evolution, our environment meaning where we were raised etc. And if said person is raised to believe that killing people of a given sexual orientation is morally right, that might only apply to that specific society. But ultimately there is no right or wrong, meaning that you won't find a universal moral judge, like a God, holding people accountable for breaking these moral rules, because they are man-made or evolved. Said in a simple way, morality is merely an agreed-upon set of rules of how to behave.
And this open up for all the fun questions like:
Stealing is immoral.
But if a starving kid steals in order to survive is that also immoral and should they be punished for it?
And questions like these