According to the article http://www.americancatholic.org/News/Homosexuality/ the 47 -bishop comitte said, "In Catholic belief, "marriage is a faithful, exclusive and lifelong union between one man and one woman, joined as husband and wife in an intimate partnership of life and love,""
The Catholic Church believes that marriage should only happen between a man and a women. However soon society and the church are going to have to accept that not all people are going to be straight and they truely belong with some one of the same sex.
The Article goes on to say, "Citing marriage's unique societal role in the procreation and raising of children, the Vatican said, "The denial of the social and legal status of marriage to forms of cohabitation that are not and cannot be marital is not opposed to justice; on the contrary, justice requires it.""
So the Church does not look to hurt or be unjust to those who want same sex marraige, they just believe that marriage should be used for the making and raising of children and since same sex people can not make children then there for they shouldnt be allowed to marry. However same sex people could addopt a child and raise it. But in the eyes of the church that is still seen as wrong.