You create a false dichotomy. If humans create something, then it in fact exists precisely as it is. Once created, it is an objective fact of reality.
It very much is not. It is something entirely subjective, even if more than one subjective opinion is involved.
As for it being arbitrary, then how can you possibly explain the widespread agreement that torturing children for pleasure is wrong? Is it coincidence? Is it magic? If many different people processing the same input produce the same output, it's nearly impossible to argue that they just made the output up.
This is problematic in many ways. The first, and most obvious, problem is that a lot of people
do torture children for several reasons, pleasure being one of them. Millions of mothers and fathers do abuse their children physically, for no other discernible reason than a desire to do so. How do
you explain
this, if there is some objective part of reality that should prevent such things from happening?
Secondly, there is a very simple explanation as to why the desire to not torture children is widespread - evolution. Humans who greatly enjoy hurting their offspring are less likely to have their genes passed on, and so partially weed themselves out. As pointed out above, however, not all do.
It's true you can't say from what objective features the output's commonality results. It obviously is partially to do with the input, since different input gives different output. But it's also mixed with many other things.
It's just like if you asked 50,000 people around the world to tell you what color the sky is right now and what the color of the grass is right now. You will see that the agreement almost conclusively proves that there is something objective about the color of the sky. (Interestingly, the disagreements tell you even more.)
Your analogy here is rather flawed. Color is a physical property that can be both explained and repeatably and impartially observed. Justice is not. And indeed, this shows.
You are implying that the 50,000 people would also tell you torturing children is wrong - but the facts speak against you. Many of them would say hitting a child is perfectly acceptable. Many believe it's beneficial to force a child to study 16 hours a day. Some think it's right to mutilate young girls permanently and painfully, so that their husband will enjoy sex more. Hundreds of similar examples can be found, and they all demonstrate one thing - that morality is entirely subjective, entirely dependent on cultural context.
There are
some moral rules common to almost all cultures. However, the ones that aren't explained by sharing information are easily explained by evolution.
Even if people had just agreed that the sky is blue and the grass is green and there was nothing inherent in the grass and the sky that made them these colors, that wouldn't render the colors arbitrary. Grass, being the color that humans agreed to call grass, would no longer be arbitrary by virtue of the fact it had been arbitrarily picked. That green was chosen for grass would then be an objective property of green, grass, and us.
I do not know what the name of this philosophy is, and I won't say that it's precisely wrong. If you want to define the words like that, it's fine - but you should realize that you are using the terms "arbitrary" and "objective" in a way that is divorced from their commonly accepted meanings.
If you say all wrong means is that people agree that something is wrong (or is based on societal factors), then once people agree that something is wrong (or establish particular societal factors), it really is wrong, since that's what you said it meant for something to be wrong. Of course, no sane person argues that wrong is whatever people agree is wrong, but to the extent that's a factor, once people actually agree, then things really are wrong.
"No sane person argues that wrong is whatever people agree is wrong"? Are you familiar with the term "poisoning the well"?
But anyway, as I've pointed out above, different cultures do indeed have very different views on what is right and what is wrong. People have agreed that female circumcision is a good thing. Other people have agreed it is torture. Which people are right? According to your claim above, both are, but this is quite inconsistent.
There's also another interesting angle. According to your politely worded claim, it's ridiculous to assume what is wrong is decided by people. Since that is the case, I assume you can provide an alternative theory. What, then, is the true source of morality?
Not quite the same thing though. 
