Why are women forbidden to Pastor?
I am assuming by your question that you understand that the Bible forbids women from being pastors and that you are looking for the reasoning behind it. The answer can be found in 1 Timothy 2:11-13 -"Let the woman learn in silence with all subjection. But I suffer not a woman to teach, nor to usurp authority over the man, but to be in silence. For Adam was first formed, then Eve. And Adam was not deceived, but the woman being deceived was in the transgression."
This passage records 2 reasons why men are to be the leaders of a local church. The first is the fact of seniority. Adam was created before Eve. The second reason is a bit more complicated, but it has to do with the word pastor and its connection to the word shepherd. A pastor is a shepherd for Peter admonished the elders of the church to "FEED THE FLOCK OF GOD which is among you, taking the oversight thereof, not by constraint, but willingly; not for filthy lucre, but of a ready mind."
According to 1 Timothy 2, Eve was deceived by the devil, but Adam knew exactly what he was doing. Although he was wrong for disobeying God, his motive wasn't selfish. His motive was love for his wife who was destined to die alone for her sin. From this passage, we can see that Adam is a type or picture of Jesus Christ and Eve is a picture of the church ("Husbands, love your wives, EVEN AS CHRIST ALSO LOVED THE CHURCH, AND GAVE HIMSELF FOR IT" Ephesians 5:25). In John 10:11, Jesus Christ said that "the good shepherd giveth his life for the sheep." Adam willing forfeited his life for his bride-Eve. I believe that this is the reason why the Lord chose to use only those of Adam's gender to pastor. If Genesis 3 had been reversed, with Adam being deceived and Eve motivated by love for Adam, I believe that the Lord would have chosen only women to pastor. The whole thing hinges on a shepherd-like quality that was seen in Adam.