This question at first seemed like a non sequitur to me, but it actually comes from an interesting place.
The Eastern Orthodox churches use the Greek word μυστήριον (musterion) to refer to sacraments, but the word actually means 'mystery', and many Orthodox would prefer the term Sacred Mystery over sacrament. Ephesians 5:31-32 says that the joining of a man and woman in marriage so that they become one flesh is a mystery, which is really about Christ and the Church, so it makes sense that it is considered a sacrament to the Eastern Orthodox churches.
The Western churches, including most Protestants, follow the Catholic Church and understand sacrament to mean a sacred symbol. The Catholic catechism says:
The sacraments are efficacious signs of grace, instituted by Christ and entrusted to the Church, by which divine life is dispensed to us. The visible rites by which the sacraments are celebrated signify and make present the graces proper to each sacrament. (1131)
The sense is not that they are mysteries, but that they are symbolic rites which effect God's grace. The Catholic Church considers marriage to be a sacrament, as Ephesians 5:32 does say that it does symbolise the joining of Christ to the Church.
Most Protestants only recognise two sacraments: baptism and communion. While Lutherans still believe that the sacraments are Means of Grace, most other Protestants generally consider them to be almost purely symbolic. The sacraments are limited to the rites which are directly instituted by Jesus himself. The Apology to the Augsburg Confession (Lutheran) says:
If we define the sacraments as rites, which have the command of God and to which the promise of grace has been added, it is easy to determine what the sacraments are, properly speaking. For humanly instituted rites are not sacraments, properly speaking, because human beings do not have the authority to promise grace. Therefore signs instituted without the command of God are not sure signs of grace, even though they perhaps serve to teach or admonish the common folk. (Article XIII)
The 39 Articles (Anglican) say:
Sacraments ordained of Christ be not only badges or tokens of Christian men’s profession, but rather they be certain sure witnesses, and effectual signs of grace, and God’s good will towards us, by the which he doth work invisibly in us, and doth not only quicken, but also strengthen and confirm our Faith in him.
There are two Sacraments ordained of Christ our Lord in the Gospel, that is to say, Baptism, and the Supper of the Lord.
Those five commonly called Sacraments, that is to say, Confirmation, Penance, Orders, Matrimony, and extreme Unction, are not to be counted for Sacraments of the Gospel, being such as have grown partly of the corrupt following of the Apostles, partly are states of life allowed in the Scriptures; but yet have not like nature of Sacraments with Baptism, and the Lord’s Supper, for that they have not any visible sign or ceremony ordained of God. (Article XXV)
Even though marriage is a symbol in which even non-Christians unknowingly still demonstrate to a limited extent the joining of Christ and the Church, and even though it is a blessing from God, and even though God did institute it, it is not a particularly Christian rite. Baptism and the passover existed before Jesus came to the earth, but he transformed them when he gave the instruction to baptise in the name of the three persons of the Trinity and to eat the bread and drink the wine to remember his death. The two sacraments recognised by Protestants directly symbolise aspects of the gospel and Christ's death on the cross.
In the gospel passages of the Last Supper and the Great Commission, Jesus institutes the two Christian sacraments of baptism and communion, but there are no similar passages where Jesus institutes marriage, nor does he transform it. Protestants do not consider a Christian marriage to be different in any way from a marriage of non-Christians. So marriage is not thought to be a Christian sacrament.