Just as marriage symbolizes the unity and devotion between a man and woman, sex symbolizes the unity of the flesh as one. Genesis 2:24 (NASB), "For this reason a man shall leave his father and his mother, and be joined to his wife; and they shall become one flesh." When a couple engages in sex, they give their whole self to their partner. They know each other intimately, without reservation. The act of sex further releases hormones that establish a strong emotional bond. Thus, the two become one in flesh and in heart. They love each other. (See the Song of Solomon 1:2-4, Ecclesiastes 9:9.)
Casual sex is a direct sin against God's expressed will. Even if we don’t understand why God gives a command, it is our duty to obey it. God's wisdom is far, far beyond ours.
In addition, your body is a “temple of the Holy Spirit” (1 Corinthians 6:19) and therefore you should keep it pure and honor it. Verse 20 states that we should honor God with our bodies and being sexually immoral makes us unclean. Ephesians 5:5 ESV – “For you may be sure of this, that everyone who is sexually immoral or impure, or who is covetous (that is, an idolater), has no inheritance in the kingdom of Christ and God.”
??Finally, marriage symbolizes the unity between Jesus and His Church. Ephesians 5:31-32 (ESV) – “‘Therefore… the two shall become one flesh.’ This mystery is profound, and I am saying that it refers to Christ and the church.” The Christians’ behavior between a husband and wife is a witness to the world about how united Jesus and His Church are. A violation of this picture is a serious matter. All symbolic pictures and types are to be carefully kept and honored.