Idealistic Argument For Objectivity of Morality

Something along those lines…

1. World is a rational place. (It makes sense)
2. What is rational can be in principle understood.
3. From 1 and 2 => the world can in principle be understood.
4. Moral judgment of a rational agent in specific situation depends on agent’s understanding of the world (including the understanding of the situation) 
5. From 4 and 3 => because the world in principle can be understood, in principle there is an ideal moral judgment (or… there is objectively right way to act, connected to the full understanding of the world)

If a person A lacks understanding person B has, A might not agree with the moral judgment of B in the concrete situation, but if A understood (or came to understand) what B understands he would agree with the moral judgment of B. As the understanding approaches ideal understanding, the moral judgment approaches ideal moral judgment.