It's very difficult to give "moral weight" to the fact that many people hold a view that we profoundly disagree with. 

Let's imagine that it is 1830 and a lot of people believe that blacks should be enslaved. Assume that we are opposed to slavery and consider it a violation of a fundamental right to liberty. How much "moral weight" should we give to the fact that some people believe it to be a justifiable institution? What implications would such moral weight have?

I know that John spoke of a prima facie obligation not to interfere with the practices of others. That sort of obligation could be overridden by other moral considerations. Perhaps we could also speak of a prima facie obligation to give moral weight to the fact that others sincerely think that something is morally good or morally bad. But, to be honest, it seems to me that a lot of limits or qualifications would have to be placed on that obligation in order to protect it from counter-intuitive examples like the case of slavery.