This week’s observance of “National Marriage Week” — an annual celebration of the importance of marriage — culminates today, on Valentine’s Day. As it concludes, I want to highlight a growing concern: the “privatization of marriage.”

The New York Times is a bellwether of trends, especially in influential circles of American society. In the past year, several articles applauded a new business: freelance writing of people’s wedding vows.

Hiring somebody to craft one’s “personalized” wedding vows, tailored to what one intends to say about marriage, usually with the peculiar quirks of one’s personality, is apparently big business. Some suggest it’s just the next step beyond speechmakers that write best men’s toasts or other’s remarks at receptions.

It’s part of a bigger trend of what I’d call “privatization of marriage.” In most states today, you don’t even have to solemnize a marriage before any public official, religious (clergy) or secular (judge, mayor). You can get a license for your own “officiant.” In a few, you don’t even have to have somebody else: you can be your own officiant. In America, not only can you bowl alone, but you can also marry alone. (While “love is love,” marriage still seems to require a numerical minimum of two.)

What’s the problem? For Catholics, a lot of things. First of all, the nature of marriage.

Even before we talk about marriage as a sacrament, people recognize marriage as a social institution. They always have. Marriage is not just something between two people. It establishes their — not just his, not just her — relationship to that society. It gives them a different status.

Protestants do not regard marriage as a sacrament. They call it an “estate” (you know, as you see in Hollywood movies, “as you enter upon this holy estate”). By that, they mean that marriage, while having nothing to do with one’s salvation, does affect one’s civil status.

But, for Catholics, marriage is more. Marriage is supposed to be a sacrament. It is part of God’s plan for human salvation and plays a role not just in our civil relationships in this world but in our preparation for the next.

I chose the words “supposed to be a sacrament” deliberately. Catholic teaching is that two baptized Catholics cannot enter into a marriage that is not sacramental. There is no such thing for a Catholic as a merely “civil” marriage. That is why, from a Catholic perspective, a “civil marriage” alone is neither a sacrament nor even a marriage. A Catholic cannot exclude his marriage from the order of salvation; a “civil marriage” is merely a simulation of marriage from a believer’s perspective.

So what does all this have to do with “do-it-yourself” (DIY) wedding vows?

If marriage is a social institution — whether in purely natural, civil society or in the supernatural community called the Church — that means that society has a common, shared understanding of what a marriage is and isn’t.

Traditional marriage vows recognize that marriage is something bigger than a couple, an institution into which these two people enter. DIY marriage vows assert that marriage is whatever these two people think it is. It’s as if every couple invents marriage from scratch.

But, as Qoheleth reminds us, there’s really “nothing new under the sun” (Ecclesiastes 1:9).

Pope Pius XI’s too-neglected encyclical, Casti connubii, identified these problems already back in 1930. People ignore the encyclical because, like Humanae vitae, it rejected contraception. But that was not its main subject. Marriage was.

Very early in the document, Pius clearly distinguished between aspects of marriage that are subject to human will and those that are not.

What is subject to the human will is the decision to marry. Whether or not Joe marries is his choice. Whether Joe marries Anne or Alice is his choice.

What is not subject to the human will is defining what marriage is, as the Pope writes:

[F]reedom, however, regards only the question whether the contracting parties really wish to enter upon matrimony or to marry this particular person; but the nature of matrimony is entirely independent of the free will of man, so that if one has once contracted matrimony he is thereby subject to its divinely made laws and its essential properties.

So, from a Catholic perspective, Joe can decide to marry Anne or Alice. But John cannot decide to “marry” Anne and Alice (polygamy); to “marry” Anne for 10 years subject to renegotiation (indissolubility); to “marry” Andy instead of Anne (sexual differentiation); and/or to “marry” Anne but refuse any children that might come of the union (openness to life). All those situations violate the nature of marriage, so any attempt to “marry” while denying those essential characteristics of matrimony invalidates a marriage.

DIY marriage vows deny that one is entering something bigger than oneself. They also run the real risk of saying (or not saying) something contrary to what marriage is. For example, there’s a reason the vows talk about “till death us do part.” We, as Catholics, hold that marriage is indissoluble. If we approach marriage intending otherwise, we invalidate that marriage. But there are lots of people today who are unwilling to say “till death us do part.”

So, as we celebrate National Marriage Week, let’s start from a critical but neglected perspective: that marriage is bigger than us, that marriage has its own meaning not subject to our redefinition or renegotiation, that marriage has its own contours and profile independently of whether we like them or not, find them convenient or not. Let’s celebrate National Marriage Week by letting marriage be marriage, not our discount imitations.