Aristotle, the great philosopher of ancient Greece, in his Politics, makes something of a romantic statement (for those who know him, something exceptionally unusual). He says that all human society begins with the love of a man for a woman. From that love comes forth a human family. From families come tribes, from tribes come cities, and from cities come nations. Therefore the human family and marriage is at the very root of society itself.
Hence it behooves society to do everything in its power to encourage marriage and the creation of families. Any policy which does not do this is suicidal, as we shall see in a subsequent analysis, it is just such a social suicide that many Western countries are experiencing right now.
Thus we arrive at the question of the definition of marriage.
After centuries of theological discussion and debate over each of its points, we are the happy recipients of one of the most well thought out positions on marriage in all of human history. Using the tools of Divine Revelation, Scripture and Tradition, along with its corollary, authentic and well-informed human reason, the Catechism of the Catholic Church gives a concise definition (1601):
Marriage is a lifelong partnership of the whole of life, of mutual and exclusive fidelity, established by mutual consent between a man and a woman, and ordered towards the good of the spouses and the procreation of offspring.
Take away anything, even one bit, of that well-considered definition and you have deformed what marriage is meant to be, both naturally and supernaturally.