It doesn't take "many people" doing something for it to be a problem. If only 1 in 50 or 1 in 100 people decided to walk around naked in public, or commit incest, or sexually abuse small children, I think that would bee too many. If laws against murder were overturned, I'm sure that not many people would become murderers. Would that make it no problem? No one is saying that ALL people only act morally under threat of punishment. The problem is that there are ENOUGH people who only act morally under threat of punishment to cause a problem.