It's an interesting topic H2H.
First consider the difference in a pastor and a missionary. There really isn't much difference except that generally the role of a pastor is to feed and pasture the "sheep" and missionaries generally are like evangelists who are sent to take the Gospel to those who haven't heard. Much of the time missionaries not only share the Gospel but also provide education, food and medical help to those less fortunate.
I don't see much of a difference in what they do so I find it interesting that women can be missionaries but not pastors.
One of the biggest differences that I see between a pastor and a missionary is that the pastor interviews for a church and is voted in or out and is paid x amount of money for wages. A missionary must come up with the money themselves or travel and get sponsors to pay his/her way to the mission field. Many churches and/or individuals sponsor them monthly, but if they have a tough month, then that missionary may or may not get their support that month.
Either way I believe that God uses each of us man or woman to accomplish His purpose and it doesn't really matter what label (pastor, teacher, missionary etc.) that person wears while serving Him.