Morals are not physical laws, nor is there any evidence humans have an innate moral sense.
Morals thus are devised solely by humanity, and are not objective in the strict sense.
If morals are subjective, humans can only create morals individually or collectively, and they are not natural at all.
Morals are what is "good" or "bad". To determine what is objectively good, you must determine what makes people happy, and happiness is certainly good. The right thing to do is what is best for the majority of individuals and what helps society make those people happy. What is morally bad is what does the opposite of what is morally good.
Anyone who claims that "Moral Subjectivism is Fact" is presupposing the adhering to reality is virtuous. They are saying
A) Moral Subjectivity is a Fact
B) You OUGHT to believe in facts.
That is a complete contradiction. A claim that requires the claim-maker to contradict them self cannot possibly be true.