This is a question I’ve asked other Catholics, and have yet to get an answer that makes sense. Before asking it, let me just say that I have recently celebrated my 26th wedding anniversary, and my prayer is that my wife and I would end our lives together in His service. I can’t even imaging being with anyone else (even in my dreams - dang it!). I hope I’ve made it clear that I am not looking for a reason to end my marriage.
Now my question - Since God instituted marriage in the Garden with Adam and Eve, how is it considered a Sacrament of the Church today? Well
Since God instituted it (and given its importance to human society even unto our salvation) - it would be harder to explain if the Church had OMITTED Matrimony as one of the seven sacraments.
But I think you may be referring to the ceremony as it is practiced today - and what difference if any there is between an OT and a NT marriage called the sacrament of Matrimony. I don’t think there is (as there is with some of the sacraments) the proof text single verse that shows *HERE *is where He instituted the sacrament of Matrimony. So WHY does the Church teach that it IS a sacrament and WHEN did it begin teaching so? Let’s investigate:
In the NT Jesus attends a wedding (details of the ceremony are not there, we join Him at the reception). But He works his first public miracle there and begins his public ministry there. The Bible “ends” in Revelation with a Bride (City/People of God) coming in glory to meet her bridegroom Christ. She has prepared herself and is now without spot or wrinkle; with the Holy Spirit she bids us to come to the wedding feast. < The Church was bound to teach about the importance of Marriage and have this teaching as one of its most important.
While the ***details of Christian weddings ***are not among the things that were included in scripture, after Pentecost the Church began acting with power. Ceremony notwithstanding the teachings of Jesus regarding permanence and God’s joining some couples together; what constituted adultery, Paul’s advice to husbands and wives (Church teaching authority in the matter) were part of the Church. By the time of Pentecost having multiple spouses had gone out of vogue (even the Samaritan woman with “seven husbands” was down to the one “not your husband”) when Jesus chided her about her seeming carelessness in the holy matter.
Baptism was a relatively new thing (one could argue it was a Jewish continuance of John the Baptist’s ministry and the pre-Pentecostal apostles) but after Jesus commissioned them to do it, it became a sacrament with life giving power.
Is being married in a church by a priest more binding upon a couple than being married by a J.P., and if so, how? <In some ways yes. The Church considers other marriages binding as well (if you’d married someone in front of a JP and were not divorced or otherwise free to marry under secular laws for example, the Church would insist upon you and your intended’s freedom to contract a marriage). As regards the **“What God has joined together …” **matter; Marrying publicly in a sacred place, with sacred promises, with the Church as a witness is binding MORE and equally as binding as the other in the lesser matters. Even though the Church teaches it is the couple that really administers the sacrament to one another (and lives it through the graces given to them by God).
Jesus being guest number one at a wedding

– is superior to not inviting Him to be there at all. Not a hard call really.
For myself, I beleive marriage was ordained by God, and, regardless of who performs the ceremony, or where it’s performed, God views all vows the same, and just as binding. I look forward to an answer! < This is broad-minded in one aspect of it (and we should respect another’s marriage and not “covet” even if the ceremony that “bound” them was not the best that could have been done). God viewing all vows as the same … hmm … you mean when the parties vowing are
equally sincere?
For some reason the sacrifices of Cain and Abel came to mind. In that case God was not pleased with Cain’s sacrifice, but was with Abel’s. He told Cain not to be jealous but to do better. < IMO a Church wedding and inviting Jesus to it is more pleasing to Him. And a better tribute to one’s spouse.

I will add a surprise ending though - in being counselled for our Church wedding, the man of the cloth that had to sign off on things seemed to want to discourage my fiance from marrying me. After one counselling session (where he spoke to her privately) she began to pull away. She called off the engagement shortly after - and while maybe it wasn’t meant to be and maybe I was saved from an iffy marriage - I always wonder if the incident would have happened to a couple who’d married civilly, then came back for the Church’s blessing.
That said, I believe there are special channels of grace in the sacrament of Matrimony that are not there in a simple civil bond.
BTW - I did look at the link CaptFun gave, but I found no explanation of why the Scriptures cited indicated marriage as a sacrament of the church.