Slightly revised; thanks to alert readers
For what it's worth, even as a Protestant, I have found the fruit of your defense of the natural law invaluable. Here is my question. If we consider marriage merely as a natural reality, prescinding from its sacramental aspects, would two amorous teenagers in the backseat of a Cadillac promising to "love each other forever" create a marriage in God's eyes? It seems to me that there must be certain conditions for a valid marital promise to be made, and these include things that the state requires for the recognition of a marriage, like witnesses and legally binding statements.
Thank you. You’re right – there is no marriage here. Let's sort this out.
First a distinction between the castle and the gate that we enter it by: Matrimony is the status of being married, a status which carries with it certain rights and duties that cannot be changed by the will of the parties; if they do not acknowledge them, then although they are in some sort of relationship, they are not in the status of matrimony. Marriage is the act of entering that status.
Now, a distinction about matrimony itself. There is only one species of marriage, but natural matrimony may also be civil or sacramental.
Natural matrimony is a complete partnership of life between a man and women, directed by its very nature toward both their own good and the procreation and care of their children. Notice that if the man and woman do not intend a procreative partnership, they are not married; they are merely in a sexual relationship. There are various other conditions too, both negative and positive; for example one cannot marry one’s sister. The act by which natural matrimony is entered is an irrevocable covenant, a free and mutual promise to accept each other for life as husband and wife.
Civil matrimony is – or ought to be -- natural matrimony which has been recognized by the state so that doubts about the existence of the marriage are removed, the rights and duties of the parties can be enforced, and the vulnerable – especially the wife and children – can be more easily protected. Hence the state requires that the covenant must be publicly registered subject to the requirements of civil law. Canon law does something similar when it requires that for the man and woman to be married in the Church, their consent must be given before witnesses and an authorized minister. The travesty of civil marriage today is that the state claims that there is a marriage when by nature there isn’t one, and that there isn’t when by nature there is. That’s what happens when we put in office tyrants who deny natural law.
Sacramental matrimony is natural matrimony which has been supernaturally lifted by the special grace of which it is the outward sign. Even apart from this grace, marriage is in principle indissoluble, but the sacrament makes it even more so, because whether or not they make use of it, the man and woman receive the grace to be bound with the love that binds Christ with the Church. Among the baptized, every valid marriage is a sacramental marriage.
Now back to those teenagers. A few minutes of passion in the back seat do not make them married. By your description -- “we will love each other forever” – what they probably think they are promising is that they will always have amorous feelings. Not only is that far short of the marital promise – for there is nothing here about joining in a complete partnership of life in the hope of children – but in fact, it isn’t a promise at all.
Why isn’t it a promise at all? Because no one can promise the impossible. Promising to have the same feelings forever is like promising to live forever. When the man and woman promise to love each other in the wedding vow, they aren’t promising to have the same feelings forever, but to persist in the commitment of their wills, each to the true good of the other. Feelings waver. Love endures.