Having sex with a nine year old child may have been common in Mohammad's time. And Mohammad himself might or might not have done it. But neither of those things would to my mind justify it. Not so long ago, it was the custom in certain parts of the US to lynch Blacks and Mexicans. Few people today would argue that it was acceptable because so many people engaged in it back then, or that we must judge the people of that time solely by the standards of that time.