Actually, as a Lutheran, they believe in Sacraments that are in the Gospel, not the entire Bible. They call Baptism and the Eucharist the “Sacraments of the Gospel” that Jesus explicitly tells that these must be done, therefore, to defend against their teaching, we use their weapons, the Gospel, as well.
And there are two key points. As the Catechism does say in CCC 1613, the Wedding Feast of Cana is important. Jesus blesses the wedding, at His mother’s request, with the first sign of things to come, turning water into the best wine, and not just some wine, but gallons of it. In the time of Jesus, weddings were 7 days long, so if the bride and groom failed to provide, the wedding would be shameful, thus Jesus gives His blessings directly in this event.
The Wedding Feast of Cana, as we mentioned that it is where He has His first sign, is the start of His ministry. It is also a type (look up typology if you don’t know about types) of the wedding of Adam and Eve. As God would begin His work creating the world with a marriage, Jesus would start His work creating our salvation with a marriage. The first commandment from God is to “be fruitful and multiply” as the first covenant that God established because marriage is the embodiment of God Himself. The love unceasingly exchanged from Father to Son, and that love manifests into the Holy Spirit, then God said “Let US make man in OUR image”. Notice the plural, and thus Man and Woman become one in order to give an offspring just like what God is.
Second, I like to use Matthew 19 in this way to really establish the link to the Gospel to Genesis because the first marriage in the Bible is the foundation of everything. When Jesus says, in Matthew 19:4, “Have you not read that He who made them from the beginning made them male and female”. Think about it, if Lutherans believe in the Trinity, isn’t Jesus just referring to Himself? Of course to avoid too much anger from the Pharisees, Jesus made that statement in third person. Just think of Jesus saying this: “Have you not read that I who made them from the beginning made them male and female”, then you start to see how Matthew 19 is really Jesus telling us that we should remember what He did in the beginning because before Abraham, before Moses, He was, and to say that the Sacrament of Marriage isn’t in the Gospels… well, they just need to listen to what Jesus says in Matthew 19 as a reminder that He established the Sacrament of Marriage in Genesis 1.