What happens spiritually when a husband and wife have a sexual relationship?
When marriage was first instituted, we read, "Therefore a man shall leave his father and mother and be joined to his wife, and they shall become one flesh" (Genesis 2:24). The phrase "become one flesh" includes the idea of sexual union, but it is not limited to only physical sexual acts. There is a joining in relationships because God had declared, "It is not good for the man to be alone; I will make him a helper suitable for him" (Genesis 2:18). It is an emotional bond between two individuals. There is also a civil joining that creates one entity from two individuals. "And He answered and said to them, "Have you not read that He who made them at the beginning 'made them male and female,' and said, 'For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh'? So then, they are no longer two but one flesh. Therefore what God has joined together, let not man separate."" (Matthew 19:4-6). In other words, the two individuals leave their respective families to create a new family.
However, the apostle Paul indicates there is a spiritual aspect to the idea of becoming one flesh. "Do you not know that your bodies are members of Christ? Shall I then take the members of Christ and make them members of a harlot? Certainly not! Or do you not know that he who is joined to a harlot is one body with her? For "the two," He says, "shall become one flesh." But he who is joined to the Lord is one spirit with Him" (I Corinthians 6:15-17).
When a person becomes a Christian, he has set himself apart, dedicating himself to doing the Lord's will. "For this is the will of God, your sanctification: that you should abstain from sexual immorality; that each of you should know how to possess his own vessel in sanctification and honor, not in passion of lust, like the Gentiles who do not know God" (I Thessalonians 4:3-5). When a person engages in sex a bond is created between the individuals. They become one on many different levels: physically, emotionally, and spiritually. If a Christian violates the law of God and has sex with someone who is not his spouse, then a union is attempted through the action of sin. How can a person who has committed his whole being to God, unite with another in sin? "This is the message which we have heard from Him and declare to you, that God is light and in Him is no darkness at all. If we say that we have fellowship with Him, and walk in darkness, we lie and do not practice the truth. But if we walk in the light as He is in the light, we have fellowship with one another, and the blood of Jesus Christ His Son cleanses us from all sin" (I John 1:5-7). You cannot be a Christian and a fornicator at the same time. "Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God" (I Corinthians 6:9-10).
The act of sex within the bonds of marriage is approved by God. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). Hence, when a married couple engage in sex, they are fulfilling their duty to one another and keeping the law of Christ (I Corinthians 7:1-9). The same action that would be an act of unrighteous lust between an unmarried couple, then becomes a righteous act between a married couple because it is done in accordance to the will of God. "Therefore gird up the loins of your mind, be sober, and rest your hope fully upon the grace that is to be brought to you at the revelation of Jesus Christ; as obedient children, not conforming yourselves to the former lusts, as in your ignorance; but as He who called you is holy, you also be holy in all your conduct, because it is written, "Be holy, for I am holy."" (I Peter 1:13-16).
As such, the joining of a man and woman in marriage to become one flesh represents the Christian joining himself to Jesus. "For we are members of His body, of His flesh and of His bones. "For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh." This is a great mystery, but I speak concerning Christ and the church. Nevertheless let each one of you in particular so love his own wife as himself, and let the wife see that she respects her husband" (Ephesians 5:30-33).