You probably don't understand what this means if you aren't Christian. A sacrament is a visible sign of invisible grace, according to Saint Augustine. Grace is the absolute Love of God, which we don't deserve and can't earn. We receive it free. So sex is a sign of God's unconditional love for us.
Sacraments also bring about grace.
Sacraments are human, earthly actions that connect us with the divine and make Jesus Christ actually present . I'm going to give you a second to think about that. Because sex is such a holy, sacred thing, abusing it is rude, disrespectful, ungrateful, and cruel to others who want to appreciate it. Misusing it is wrong, quite simply. The love between a man and a woman must be open to life, because God made it that way in Creation.