From Archbishop Robert J. Carlson's "Before the Cross" article for the last Review edition:
God's law in the Old Testament is clear and unambiguous: You shall not kill. Jesus is even more demanding: Every one who is angry is liable to judgment.
Sins against the Fifth Commandment are easy to commit. Any time we think, speak or act out of anger or hatred or jealousy or revenge, we abuse God's commandment that we respect His most precious gift, the gift of life — especially human life.
Human life is sacred because, from its beginning until its natural end, it involves the creative action of God. The Fifth Commandment forbids direct and intentional killing as gravely sinful. God alone is the Lord of life. No one has the right to end arbitrarily what God has begun, and sustained, through the gift of His love.
You cannot be "pro-choice" (pro-abortion) and remain a Catholic in good standing. That's why the Church asks those who maintain this position not to receive holy Communion. We are not being mean or judgmental, we are simply acknowledging the fact that such a stance is objectively and seriously sinful and is radically inconsistent with the Christian way of life.