Moral realism or moral objectivism is a the view that moral values and statements are facts that are independent of the person who is uttering them.
For example the statement "Killing is bad" is True regardless of who is saying it, and even if nobody believes it. Similarly the statement "Rape is acceptable" is False regardless of who says it or who believes or doesn't believe it.
Now lets assume that one wants to defend the objectivity of morals without resorting to a religious argument, i.e. one wants to consider statements like "Killing is bad" objectively true without having to say "because God said so" or "because it is bad Karma", etc...
It seems to me that any secular argument that can be provided for the objectivity or morals can be immediately refuted by the fact that morals seem to change over time. Slavery was considered acceptable for most of human history, but now it is almost universally rejected. Pre marital sex was considered bad in most western societies for along time but is now accepted by many people in the West.
If someone is trying to prove that "Killing is bad" is an objectively True statement, another can respond "how do you know that in the future people won't find gratuitous killing acceptable? After all, a man and a woman living together without being married would have been just as horrifying to a person from 16th century as murder is now".
How can any argument for the objectivity of moral statements be defended against such a historical argument?