When the Church states that marriage is the union between one man and one woman, she is not teaching it out of hatred or discrimination. The Church never “hates” gay people. In fact, she has great compassion for people who are struggling with their sexuality and gender identity. The Catechism of the Catholic Church states that “any sign of unjust discrimination” toward homosexual people is to be avoided. Jesus, who heads the Church, loves all people—especially those whom society scorns, including people who are attracted to the same sex.
For Catholics, marriage is not only about “intimate association” and “the hope of companionship,” but it is also linked intrinsically to the procreation and education of children. Today, this concept seems foreign to many people. The “contraceptive culture” has made us believe that the main function of our reproductive systems is not for reproduction, but for the pursuit of pleasure or intimacy. Pope Paul VI’s encyclical Humanae Vitae teaches that sex within marriage has two purposes: the unitive and procreative. If we remove one of these purposes—or both, we are left with a physical union of bodies, without any transcendent meaning. Sex and babies are so separated in the modern mind that it is difficult to explain why and how marriage is more than two people who love each other; but that it is a conjugal union that exists in part for the creation and education of children.
The Church cannot change the definition of marriage. She continues to teach that marriage is not for two men or two women; because such relationships do not result in the birth of children or allow a child to be raised by his/her biological mother and father. This is not at all a reflection of hatred or bigotry for gay people.
(Reference: http://www.catholicnewsagency.com – “What the Catholic Church does (and does not) teach about same-sex marriage” by Jennifer Manning; Catechism of the Catholic Church #2358)