It might be worth reading the beginning of Matthew 23 to see what Jesus was getting at in verse 9:
1Then Jesus said to the crowds and to his disciples: 2"The teachers of the law and the Pharisees sit in Moses’ seat. 3So you must obey them and do everything they tell you. But do not do what they do, for they do not practice what they preach. 4They tie up heavy loads and put them on men’s shoulders, but they themselves are not willing to lift a finger to move them.
5"Everything they do is done for men to see: They make their phylacteries wide and the tassels on their garments long; 6they love the place of honor at banquets and the most important seats in the synagogues; 7they love to be greeted in the marketplaces and to have men call them ‘Rabbi.’ 8"But you are not to be called ‘Rabbi,’ for you have only one Master and you are all brothers. 9And do not call anyone on earth ‘father,’ for you have one Father, and he is in heaven. 10Nor are you to be called ‘teacher,’ for you have one Teacher, the Christ. 11The greatest among you will be your servant. 12For whoever exalts himself will be humbled, and whoever humbles himself will be exalted.
I don’t think it is the use of the title father that is a sin, it is the misuse of it. If a man is proud of having this title, then his lack of humility is a sin.
“For whoever exalts himself will be humbled, and whoever humbles himself will be exalted.”
(verse 12)
It’s a warning to you and me to not make a priest (or a rabbi, or reverend, or pastor, or whoever) someone you love and obey more than God. It’s a warning for a priest (or rabbi, or reverend, or pastor, or whoever) to not become proud and think of himself as better than those whom he is to serve.
I could be wrong, but that’s what seems at the moment to be the the truth to me.