I have heard that line of reasoning and I do not agree with it. I think it is poorly reasoned to say the least. I also think that Jesus was teaching a lesson about sitting in judgment of others. Before we condemn another we must first look at our own sins and remember that we are no better off than the person we are about to stone.
Does that mean that I think the death penalty is never warranted? No. There are times when is the correct action. It is not the correct action for sexual immorality such as affairs and homosexuality and fornication.
how do you support that scripturally?