Via Catholic Exchange:
With the same nuptial imagery, and in accord with the natural moral law, the church recognizes that marriage is between a man and a woman, for life and for the sake of family. Marital union is based on a man and a woman becoming “two in one flesh.” Without such self-giving union, marriage is impossible. A marriage that is not or cannot be consummated in sexual union is recognized as invalid in both church and civil law. Genuine love and deep friendship are possible without two persons becoming “two in one flesh,” and love and friendship should always be respected and encouraged. But sexual activities separated from the context of the marital union are inconsistent with the order of human nature itself.
To speak of “the order of human nature itself” becomes progressively more difficult and less convincing when gender is regarded as a purely human construct, a cultural invention, and not something given in nature. Nature itself has now been mostly reduced to a field for scientific experiment and human control. Even biological differences are to be manipulated for economic profit and according to personal preferences. Whatever restricts personal choice is politically and socially unacceptable. So two men should be able to marry, if that is what they want; and women should be candidates for ordained priesthood, if that is what they believe. Women and men are interchangeable at will.