Man is subjective. Morals laws are not. Just because not everyone discovers them or accepts them does not negate them. You are literally arguing for moral relativity. For instance, regardless of what societies have deemed acceptable, it was always wrong to treat human beings as property to be disposed of at the will of its owner, right?
We accept this as "moral" today, we didn't in 1861. So no, the history demonstrates it wasn't always unacceptable.
So you're trying to claim that morals are there and we've just not accepted them yet... but then, for years it was illegal and immoral to be a homosexual. Today, they are a protected class... is THAT the TRUE moral we have found? Twenty years ago, there were two genders... today there are over 100 purported gender identities... new moral discovery we stumbled upon?
I'm not arguing moral relativism, I am arguing morality is subjective and changes with culture and time. Back in the 1800s, young girls were married by age 14 or they were considered spinsters and ostracized. Today, that's sexual child abuse... but there are people out there lobbying for us to lower age of consent. Maybe 14 is the natural moral age when girls are able to consent? They are capable of reproduction, so mother nature doesn't seem to object... laws of nature aren't wrong about morals are they?
Now, I am sure you will make some argument about girls not being capable of consent at 14 but you don't know every 14 year old girl. That's an arbitrary standard you've set based on your sense of morals. How do we know that shouldn't be age 25? Maybe THAT is the TRUE natural moral standard? What makes age 16 or 18 the TRUE moral boundary? How do we KNOW this?