Honoring one’s father and mother is a grave obligation, which this person would have been ignoring with full knowledge and consent of the will in his mind when he said “I will not.” Jesus doesn’t seem to think this is a concern at all here because the son ended up doing the right thing.
Ok lets presume it was grave matter for the son. But even if not…still a sin is a sin.
Lets look again at the full story:
Jesus is addressing the Chief Priests and elders of the temple…after they have challenged him on what he was doing…
Matt: 21:23-32 RSV CE
28 “What do you think? A man had two sons; and he went to the first and said,
Son, go and work in the vineyard today.' 29 And he answered, I will not’; but afterward he repented and went.
30 And he went to the second and said the same; and he answered, `I go, sir,’ but did not go.
31 Which of the two did the will of his father?” They said, “The first.” Jesus said to them, "Truly, I say to you, the tax collectors and the harlots go into the kingdom of God before you.
32 For John came to you in the way of righteousness, and you did not believe him, but the tax collectors and the harlots believed him; and even when you saw it, you did not afterward repent and believe him.
The parable is directed towards the chief priests and elders…
The grave sinners …were getting into the kingdom before them! Harlots (prostitutes)…how??? by repenting…
The son ‘repented’ and went and did what his Father wanted…he repented for what he had chosen…
Just as I must do if I choose not to go to Mass…but later change my mind.
Jesus is not saying “all was fine” with the son…that he did not need to repent…(that he did not sin in his choice) …but rather he did…just like the prostitutes…
he is using this story …as directed to the Priests etc of the Temple…pointing out to them…hey these guys sinned…the prostitutes etc…but now they heard the call to repent and the have…and are getting into the Kingdom !