This is a common question as some folks seek to redefine marriage. Historically, since the dawn of creation, mankind has known that there is something special and unique between the joining of a man and woman in marriage. This bonding is an unbreakable covenant (not a contract) which unites families (tribes) and unites us in a very special way to the family-hood of God. It was never seriously questioned nor was it proposed that other types of unions would be called marriages, or that other relationships ever be equal to this special union.
Now though, as various vocal groups want their “rights” to be married, Catholics are challenged to explain why this isn’t possible. In other words, we are faced with having to answer the question, “Why can’t 2 (or more) people who love each other get married?”
There are many good answers for this, but here are some thoughts on the topic to consider. First, realize that there are various types of loving relationships, most of which would never enter into a marriage covenant. As an example, I love my parents, but that doesn’t mean we should call this love a marriage. Likewise I love my siblings, but again our relationships are not marriages nor should they be simply because we love each other. I love good friends, but again I have no desire to consecrate that love in a marriage relationship. We simply don’t nor should we marry anyone simply because we “love” that person.
Second, words have meaning. Historically as I’ve stated, marriage has a certain meaning. While we can change the meaning of words, in order for the meaning of a term to have constancy and retain its meaning, it’s foolish to think the meaning of words can be changed simply because “we want to change them.” There are truths in life, and living in a stable, functioning, predictable society requires adhering to these truths and the dependable meaning of the words we use.
Third, there is a purpose to the meaning of marriage. Again historically, and for believers theologically, the special union of a man and woman unites families and continues the family heritage by bringing children (if the couple is fertile) into this special relationship. Since children do best and have a right to a mother and father, bringing children into any other relationship is wrong as it hurts the children by depriving them of their rights to a normal, natural, family relationship (of course there are exceptions for such things as single parent adoptions, but I think we can all admit even this is not ideal as it deprives the child of being raised by both a male and female, and various problems can arise when a household is devoid of both genders as parents.)
For another reason why “love” simply doesn’t automatically equate to marriage, we simply need to look to nature to see how animal “couples” exist. As a veterinarian, I know of no animal species where the natural way to interlock genitals and reproduce involves 2 animals of the same gender (again I’m speaking of higher animals species here, not lower-life creatures whom God designed to reproduce in an asexual or bisexual way.) It’s simply not possible for 2 males or 2 females to mate and bring forth new life. Since we are a higher form than animals (although sometimes I question this based upon our behaviors!,) our reproduction is the same as the other mammalian species and we should not go against nature (God) simply because something feels good; doing so violates the natural law as well as God’s law. And the very fact that 2 men and 2 women cannot mate and have natural vaginal sexual intercourse clearly shows, through their bodies, that this union, which belongs to marriage, cannot occur. The Catholic Church teaches that the sacrament of marriage is made whole when consummated; 2 adults of the same gender cannot consummate their relationship and thus the relationship can never be a sacramental marriage (nor really any kind of marriage once we understand what marriage means and what it takes to be married.)
Another glaring question is why a “marriage” has to be limited to only 2 people? Why restrict (discriminate) those who want 3 or more to enter into a “marriage” relationship? And when we say 2 people who love each other should be allowed to marry, does the nature of the 2 people matter? Can I, a 60 male adult, marry a 15 year old girl? Why not? Why discriminate based upon age? Can I marry my sister? Why not? There are certainly legal benefits for 2 people to marry, so if my sister and I were both single and available for marriage, why couldn’t we marry and take advantage of tax benefits, social security benefits, etc.? We don’t have to sexually consummate the relationship, but why can’t we marry so we can both benefit financially and socially? Isn’t this discriminatory?
Finally, and I would argue most importantly, let’s take a hard look at the word “love” (before our broken humanity redefines that term as well.) Love means many things, including a deep affection for another, or a strong attachment to another. However, the Catholic definition of love goes much, much deeper. As Christ teaches, love hopes for and seeks the ultimate good of the other person. “Love,” writes Pope Benedict, “is 'divine' because it comes from God and unites us to God. It makes us a 'we' which transcends our divisions and makes us one, until in the end God is ‘all in all'.” St. Thomas Aquinas defines love as "willing the good of the other." While there are, according to the Greeks, 4 kinds of love, those who seek to redefine marriage such that 2 (or more) people of the same gender (or mixed genders for a polygamous union) do so more for the eros/erotic love of sexual desire than the agape love of dying to oneself for the good of the other. If sexual union between 2 people of the same gender was anything more than erotic love and actually sanctioned by God, that might be different. But since this is not the case, it is always true that any sexual union between anyone other than man and woman in marriage is not by definition agape love, since we are not willing the good of the other if our actions cause that person’s ultimate good to be in jeopardy now and for eternity. While none of us can judge a person’s soul, we can say that if 2 people knowingly engage in immoral activity, that is certainly not willing the good of the other person as that activity might imperil that person’s good in eternity.
No matter how you slice it, “marriage” has to be between 2 people of opposite genders for it’s true meaning to occur, and to truly will the best for the other (for true love to flourish and for God to grace that union which strengthens the love between the couple.) Yes, any number of people of one or both genders can engage in orgiastic sexual behavior, temporarily or for life, but this is not marriage. It is simply for physical pleasure which is fleeting and ultimately damaging to all involved. Satan attacks us at our most primal level, that of our bodies, our genders, and our families.