Eireann,
I know this example has been used before but it is very handy, so I will use it.
According to your ideas of what morality is, and what shapes it, you say that it was a moral act for Germans to kill Jews during the rule of the Nazi's when they were in power in Germany. You in fact would have agreed with it, and possibly participated in it, according to your statements here, had you lived there during that time, for you would have found nothing wrong with it. Public opinion and peer pressure were all in line for you to accept it.
Nazism, according to your definition, is a strong example of the public working to change moral values. Somehow I will just about bet that you will disgree with my conclusion that you would have participated and agreed with the extermination of the Jews, but your whole argument here says that you would. You make public opinion your whole definer of morality, and public opinion there was in favor of eliminating the Jews.
The example of the Nazi's is only one piece of evidence that morality can't be made by public opinion, just the public's opinion of it. When the Nazi's were killing the Jews it was wrong then, and it still wrong. Public opinion not withstanding.
Would you like another example? In the history of the US we had public opinion solidly behind the cheating of the Indians out of their lands. We lied, cheated, and the like, to accomplish what we wanted. Now, according to your definition, this was moral because public opinion was behind it. But, yet I'll bet you'll agree that the way we treated the Indians was far less than an example of good moral behavior.
I have given at least two examples that show how bad an idea it is to maintain the idea that morality is nothing more than public opinion and peer pressure. These two examples alone show that there is something more than public opinion behind what is moral and what is immoral.