"There is one thing man and woman cannot do, however, and that is to change the language of the body itself. This language is inscribed in our very being by the Creator."
I took the following from an interview with Fr. Walter Schu, L.C., whose life work is dedicated to the study of John Paul II and the Holy Father's catechesis on what is called the Theology of the Body. I think it is important for Catholics to understand that the issue of same sex marriage is fundamentally a faith and morals issue which has been politicised by gay activists and media. Faithful 'gay-Catholics,' frustrated with prevailing narratives about homosexuality, seem to want to disassociate themselves from any Catholic effort to promote traditional marriage, considering it excessive focus on political issues which muddies the waters of a developing gay-Catholic-spirituality. Likewise, Catholic gay-activists argue that the bishops should not be meddling in political/legislative actions that promote same-sex marriage, claiming they have no authority in that realm and they will only alienate countless souls from the Church. That is nonsense of course, since as I said, the issue is one of faith and morals, and is therefore very much something within the bishops authority and realm of responsibility.
26. Obviously, another major issue is the understanding and definition of marriage as between a man and a woman. Can you discuss how Theology of the Body helps us understand why same-sex “marriage” is not possible?
Same-sex “marriage” is not possible, because marriage is not simply a human institution that we can modify or redefine at will. Marriage was instituted by God himself, when he created the first man and woman before the dawn of recorded history. Marriage entails the exclusive gift of one man to one woman, and vice versa, in their entire person, their whole “self,” over a lifetime. This “law of the gift,” this call to self-giving love is inscribed in our very nature, which John Paul II calls the “language of the body.” Husband and wife, by their total self-giving love in marriage, bear wonderful fruit in bringing forth children, in creating a family, a new community of love, which confirms and deepens their own communion of life and love. This fruitfulness images the fruitful, self-giving love of the Blessed Trinity, the communion of persons in love that is God himself: Father, Son, and Holy Spirit. Just as Shakespeare’s works raised the English language to new heights, the language of the body is a beautiful poem of love.
There is one thing man and woman cannot do, however, and that is to change the language of the body itself. This language is inscribed in our very being by the Creator. There are two and only two ways to be a human person: either as a man or a woman. The gift of self between husband and wife is expressed and made incarnate in the act of conjugal union, in sexual intercourse between the spouses, when the two become “one flesh,” with the intrinsic fruitfulness that is inscribed in this act.
Same-sex partners are incapable of a true act of sexual union. They cannot become “one flesh” and cannot bring forth children. This reality reflects the fact that they cannot live out that exclusive gift of one’s entire self in married love, which only a husband and wife are capable of, due to the language of the body inscribed in our very being as man and woman created in the image of God.
The fruitful gift of one’s entire self to the other in love that takes place in marriage is possible due to the authentic complementarity between man and woman. This complementarity is not only physical, but also extends to the sphere of the emotions and even to the very depths of the person. This complementarity is patently lacking between same-sex partners. - Source
Fr. Schu celebrates Mass at my parish in Minneapolis whenever he is in town visiting his mother.