This one has always puzzled me a bit, because the Bible does allow divorce for adultery (marital unfaithfulness).
There is a difference between being divorced and annulled. But even being divorced and thus excommunicated, one can have that overturned if one is willing to go through the hoops and show his/her devotion to the church. There are some nominal feels for classes and the like, and I have heard many Christ-aphobes claim that it costs tens of thousands of dollars to have this done (their excuse to bash the church) but I have yet to see any proof of this. I think our church charges around 200 bucks for the classes, and they last for months and cover everything from catechism to marriage relationships, etc. Much cheaper than marriage counseling in any event.
They require the classes for both man and woman, so as to help prevent another divorce I imagine (in the case of remarrying).
In the King James version. That is not the Bible Catholics use.
Marriage in the Church is a sacrament of the Church and as such it can only be dissolved by the Church. If they don't allow the divorce/dissolution of the marriage then you are still married. If you civilly marry another person in the Church's eyes you are essentially living in sin. Until you correct/stop that on going sin you aren't supposed to take communion.
I don't think the same thing applies if the first marriage is not done as a Catholic religious ceremony. If you are married by a Justice of the Peace, a minister of a different religion, etc I don't think you have the same problem in taking communion or getting married again in the Church.
It all comes down to the interpretation and misinterpretation of the Greek word "porneia". This Catholic Apologetics site explains it pretty well.