One would assume that if you must be at least 18 years old to marry validly in the Catholic Church in the United States, you would have already received baptism, confirmation, holy Eucharist and confession.
The Code of Canon Law makes this statement: “Catholics who have not yet received the Sacrament of Confirmation are to receive it before being admitted to marriage, if this can be done without grave inconvenience” (Canon 1065.1).
The question is: Why is it fitting to receive confirmation before marriage? Because confirmation, if received worthily, brings with it the grace and strength you need to fight the good fight and live as a Christian. With confirmation the gifts of the Holy Spirit are strengthened: wisdom, understanding, knowledge, counsel, fortitude, piety and fear of the Lord. All of these are helpful for married life.