Excommunication

I had a conversation with a friend a few weeks ago on the topic of excommunication. It seems like quite the tricky topic now that I think about it. On the one hand there are places in the Bible that seem to justify the practice: "If your hand causes you to sin cut it off," among others. Yet, if the church is supposed to be about forgiveness and bringing all people closer to God, how can it purport to expel some of them?

Furthermore if we are to accept this form of censure, what does it say about the Church that it will excommunicate someone like Rev. Roy Bourgeois for "ordaining" women but not do so to the many priests who were involved in the sex abuse cases? Is ordaining women worse than sexually abusing members of the congregation in the eyes of the Church? Does it jeopardize the teachings of the faith more? Is it more inherently opposed to Catholic Doctrine?