If one says that ELCA affirms sinful behavior as not being sin, then that is simply telling the truth, not slander.
I know an ELCA member who supports ordaining gays. She said, “The church should change, to conform to society.” I said, “No, society should change, to conform to the Bible.” She wasn’t able to argue that I was wrong, about that sentence. We argued for about a half-hour, and neither of us changed our mind.