I don't know which passage/s you are referring to. I know of a hadith which depicts a scene of the Day of Judgment, where Adam, Noah, Abraham, and Moses all confess to sins, but Jesus does not mention any sin. As far as I know, we have no mention of Jesus committing any sin in our Scripture. We also have a hadith that says that when every child is born, Satan pokes the baby, and that the only two that did not get poked by Satan were Mary and Jesus. However, the generality of the statement that all sons of Adam sin, and our belief that Jesus is a son of Adam, would lead me to believe that Jesus was not completely sinless, though definitely, incomparable to people like us, who are very sinful. I know this would not add up in your mind, since you view all sin as equal. For me though, we view that some ba deeds are worse than others.
First of all, the
lusting thing is not true. That is a disgusting exaggeration. And that verse that mentions that he was hiding some feelings is strong evidence of Muhammad's honesty in transmitting the revelation of God. It must have been very embarrassing for him to tell everyone that he had been attracted to her, yet God wished to reveal what he could have easily kept secret in his heart. He would never have fantasized about anyone, or lusted, or done anything immoral nor indecent. It is normal human nature to find certain people attractive, regardless of how pure and moral you are. Muhammad was a human, but, I believe he was the best human, and an example for us all.
As for the other things, those things go back to the culture of the time, and more importantly to the wisdoms for which God ordained these things. Marriage is a BIG responsibility. It's not like, "Oh, lucky me, I have a dozen wives." Each one of Muhammad's wives was different, and the things we learn from each are unique. Women played a big role in the transmission of Islamic knowledge to us, ESPECIALLY the child bride you are referring to. The details of Prophet Muhammad's personal life were related to us by her. She had an amazing mind. She lived for a long time after the Prophet's death and continued to teach both men and women for decades afterwards. I do not consider any one of the things you mention as sinful behavior. In fact, it was per God's command that he married many wives and it has served Islam greatly. Also, I wouldn't believe that David lusted over anyone either. Prophets don't do that sort of thing.
And as for polygyny, it's not even God who forbade that for you Christians, as far as I know. That rule was made way after Christ, correct? Do scholars have the authority to change God's rules like that?
Allah says in the Qur'an (translated) about the Christians:
They have taken their scholars and monks as lords besides Allah , and the Messiah, Jesus the son of Mary. And they were not commanded except to worship one God; there is no deity except Him. Exalted is He above whatever they associate with Him.
When this verse was revealed, Adi ibn Hatim, who had been a Christian, came to the Prophet Muhammad and objected. He agreed that they had taken Jesus as Lord, but said that Christians never worshiped their scholars or monks. The Prophet Muhammad asked him,
"Do they not forbid what Allah has permitted and do you not then forbid it (to yourselves), and do they not make permissible for you what Allah has forbidden, and do you not then make it permissible (to yourselves)?" I replied: "Certainly!"He
said: "That is how you worshiped them." (Narrated by At-Tirmidhi)