The concept of Natural Law puts the concept of sex for procreation in an unique position where it relates to that special relationship between man and women. It says that the natural design that has made procreation between a man and a woman the natural order of things. It is that natural order that cannot be broken and must be protected.
The Christian thinking takes the natural order to a new level: The Divine Order. Man and women have been designed by God for the purpose of creating new human beings for His glory. That special purpose has been reinforced by that special relationship with marriage. Catholics have sanctified that special relationship with the sacrament of Matrimony. It is a divine creation by the author of the universe that cannot be amended by man. The gay community has other ideas about God. They will be judged by that rebelliousness.
Not all cultures have religion, but they all have marriage, and it always involves a man and a woman. For instance, even with polygyny and (much rarer) polyandry, the plural wives aren't married to each other, nor are plural husbands married to each other. It's always the male-female bond that consttutes marriage. Isn't that so?