We are talking about a knowing rejection. This is a rather grim message, unpleasant, I would say politically incorrect, and most people today would say it is no way at all to attract people back to their religion or towards religion. But nonetheless these warnings are a regular pattern in his teaching. Part of what Jesus is saying is that God will judge us according to the way we honestly see things. But while we might be able to fool other people and sometimes even ourselves we are not able to fool God.
Real honesty is an important criterion.
We are not talking about breaking a taboo. When you break a taboo it does not matter whether you understand it or whether you do not. You are still in trouble. That is not what following Christ is about. Our Lord s call to faith and goodness is important.