I am not questioning the idea, merely wondering. What makes marriage a sacrament?
I see why it would have been a sacrament in the past. When all medieval territories had different rules, many were conquered every few months, and rulers were above the law, having the Church regulate marriage was definitely the least confusing way to do things.
Is the idea that it’s a “sacrament” left over from when church was law and establishing matrimony as a “sacrament” kept track of things better? Or why else?
The Council of Trent
On The Sacraments In General
CANON I.-If any one saith, that the sacraments of the New Law were not all instituted by Jesus Christ, our Lord; or, that they are more, or less, than seven, to wit, Baptism, Confirmation, the Eucharist, Penance, Extreme Unction, Order, and Matrimony; or even that any one of these seven is not truly and properly a sacrament; let him be anathema.
Nice quote, but that’s not an answer. As the French say, on n’attrape pas les mouches avec du vinaigre.
OP: Marriage is a Sacrament because it’s part of God’s great design for man and woman. A careful look at passages like Genesis 2: 18-24 and Matthew 19: 1-9 makes it clear that this was part of both the Old and New Covenants. You might also like to have a look here:
Catechism of the Catholic Church1613 On the threshold of his public life Jesus performs his first sign - at his mother’s request - during a wedding feast.105 The Church attaches great importance to Jesus’ presence at the wedding at Cana. She sees in it the confirmation of the goodness of marriage and the proclamation that thenceforth marriage will be an efficacious sign of Christ’s presence.
1614 In his preaching Jesus unequivocally taught the original meaning of the union of man and woman as the Creator willed it from the beginning permission given by Moses to divorce one’s wife was a concession to the hardness of hearts.106 The matrimonial union of man and woman is indissoluble: God himself has determined it "what therefore God has joined together, let no man put asunder."107
Gaudium Et Spes (excerpt N. 48):
Authentic married love is caught up into divine love and is governed and enriched by Christ’s redeeming power and the saving activity of the Church, so that this love may lead the spouses to God with powerful effect and may aid and strengthen them in sublime office of being a father or a mother. (Second Vatican Council, Dogmatic Constitution on the Church: AAS 57 (1965), pp. 15-16; 40-41; 47) For this reason Christian spouses have a special sacrament by which they are fortified and receive a kind of consecration in the duties and dignity of their state. (Pius XI, encyclical letter Casti Connubii: AAS 22 (1930), p. 583) By virtue of this sacrament, as spouses fulfil their conjugal and family obligation, they are penetrated with the spirit of Christ, which suffuses their whole lives with faith, hope and charity. Thus they increasingly advance the perfection of their own personalities, as well as their mutual sanctification, and hence contribute jointly to the glory of God.
Brendan points out one of the key concepts that you are missing. Namely that sacraments are a vehicle through which God provides grace.
Marriage exists regardless of the state recognizing it or not. The sacramental nature of marriage is wholly independent of the state.Yes, the Church does defer to the state on the legal requirements being met, but as a concept marriage predates organized government.
You must understand that a sacrament is not just for this life, but rather to prepare us for the next. As one of the two sacraments of vocation marriage is how we live out spreading the gospel. In particular marriage provides the graces to strengthen a husband and wife as they spread the gospel to their children.
From the context, function, and form of these, I’d say “No, and yes” respectively.
God does not save us through marriage precisely the same way we are saved through Baptism, Holy Communion, and Confession, though as a married father I recently said to my wife “I get it”.
I get why it is a Sacrament; I understand Humanae Vitae; sudden clarity.
There is a unique type of support and love through one’s spouse that I’d call actual grace; you function as a visible sign and metaphor of God’s love and creative power to the world; you actually cooperate with God’s creative plan through the marital act as it is sometimes called.
Ephesians 5 draws the connection between Jesus’ love for His Church and how man should love his wife and also why:
25 Husbands, love your wives, as Christ also loved the church, and delivered himself up for it:
26 That he might sanctify it, cleansing it by the laver of water in the word of life:
27 That he might present it to himself a glorious church, not having spot or wrinkle, or any; such thing; but that it should be holy, and without blemish.
The above requires Grace.
28 So also ought men to love their wives as their own bodies. He that loveth his wife, loveth himself.
29 For no man ever hated his own flesh; but **nourisheth and cherisheth it, as also Christ doth the church: **Grace to aid man’s imitation of Christ.
30 Because we are members of his body, of his flesh, and of his bones.
31 For this cause shall a man leave his father and mother, and shall cleave to his wife, and they shall be two in one flesh. Again, ties with Gen 2
32 This is a great sacrament; but I speak in Christ and in the church.
Scripture never fails to identify marriage as the one and only institution described “In the Beginning…”
All she asked was: What makes marriage a sacrament? and Is the idea that it’s a “sacrament” left over from when church was law and establishing matrimony as a “sacrament” kept track of things better? Or why else?
Well it’s canon law and one I believe that can’t be tampered with, so it’s a sacrament because it’s canon law and it’s still relevant because it’s canon law.
No argument there. I was specifically addressing the OP’s statement:
*Is the idea that it’s a “sacrament” left over from when church was law and establishing matrimony as a “sacrament” kept track of things better?
*I was more trying to speak to the fact that marriage is a natural institution established by God long before states needed to “keep track of things”. I would say the order was:
*]God established the natural state of marriage
*]The state recognizes marriage
*]Christ raised matrimony to the dignity of a sacrament
*]The state completely messes up what marriage is.
The original marriage was without divorce. It existed for Adam and Eve when they were just. The Jewish law was not that, as Christ explains, related in Matt 19. The state of justice is necessary to receive the increase of grace of matrimony which certainly was available from the time of Christ’s salvation of Mankind. It seems that Christ restored original marriage of Adam and Eve. It may be that the state has never recognized original marriage. What do you think?
Christ speaking in Matt 19:  Who answering, said to them:
Have ye not read, that he who made man from the beginning, Made them male and female?
And he said:  For this cause shall a man leave father and mother, and shall cleave to his wife, and they two shall be in one flesh. Therefore now they are not two, but one flesh. What therefore God hath joined together, let no man put asunder.
 They say to him:
Why then did Moses command to give a bill of divorce, and to put away?
 He saith to them:
Because Moses by reason of the hardness of your heart permitted you to put away your wives: but from the beginning it was not so.  And I say to you, that whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery: and he that shall marry her that is put away, committeth adultery.
 His disciples say unto him:
If the case of a man with his wife be so, it is not expedient to marry.
 Who said to them:
All men take not this word, but they to whom it is given.  For there are eunuchs, who were born so from their mother’ s womb: and there are eunuchs, who were made so by men: and there are eunuchs, who have made themselves eunuchs for the kingdom of heaven. He that can take, let him take it.
God communicates much to us through the grace in marriage. Without this revelation from God we can’t really understand love, or much about God for that matter. Mainly because the Sacrament of Marriage is the place we Christians learn the important difference between Eros and Agape.
Other ways to express this,
When the Catholic Church teaches that marriage between two baptized persons is a sacrament, it is saying that the couple’s relationship expresses in a unique way the unbreakable bond of love between Christ and his people. Like the other six sacraments of the Church, marriage is a sign or symbol which reveals the Lord Jesus and through which his divine life and love are communicated.
As a sacrament, marriage contains two key elements, according to Archbishop Sheen, which correspond to the dual nature of all sacraments: the visible and invisible, natural and supernatural.
“One is very visible and evident. It is the exchange of consent which is signified not only by the joining of hands, but also by the words of consent. And this is witnessed by a priest. There is the invisible grace, also, which is communicated for their married state of life, and because this grace symbolizes another marriage—the marriage between Christ and His Church. That is the meaning of sacramental marriage (and it is found throughout the Epistles of Paul).”