For solid, Christian marriages, it is important to understand the Bible's teaching regarding sex; and not only regarding the Bible's prohibition of sex outside marriage, but the Bible's teaching on sex within marriage. The Bible has important and pointed instruction about this part of a husband's and wife's behavior.
Both the Old and the New Testaments have something to say about sex in marriage. In Proverbs, King Solomon instructs his son about sex. In the first letter to the Corinthians, Paul answers questions put to him by a young, serious-minded church, about problems in marriage - - questions about fornication, about marriage and divorce, about fathers giving away their daughters in marriage and, yes, questions about sex in marriage. God calls the church to teach her members about sex.
(This says something practical, by the way, to Christians today: If we have questions about any aspect of marriage, we should not go first to all kinds of books, but to God's Word, and to God's appointed servants who are given by God for help in these matters.)
The Apostle Paul gives instruction in I Corinthians 7 about sex in marriage, and points out that it is Necessary, Blessed, and God-Glorifying.
Sex in marriage is NECESSARY "to avoid fornication". This is a very realistic approach to sex and marriage. Realism is that we acknowledge the strong desire created in man and woman by God, a lesson the child of God forgets to his harm.
First, the apostle Paul says, "to avoid fornication, let every man have his own wife, and let every woman have her own husband" (verse 2). All by itself, this overthrows the teaching that sex in marriage is only for producing children. Second, Paul says, "it is better to marry than to burn" (verse 9). Paul is not teaching that one will burn in hell (although if one gives in to his sexual urges outside marriage without repenting, he will). Paul means that it is better to marry than to burn in one's lusts. Marriage is the remedy for that.
Sex in marriage is also necessary because it is a "debt" one spouse owes to the other. I Corinthians 7:3 says, "Let the husband render unto the wife due benevolence: and likewise also the wife unto the husband." "Due benevolence" is God's careful way of referring to the sexual obligation of one spouse to the other. Christian husbands and Christian wives owe it to their spouse to give themselves to each other physically. Because this is Jesus' command, you owe it to Jesus to give yourself to your spouse.
In the next verse, Paul explains how this can be called a debt. "The wife has not power (really 'authority') of her own body, but the husband: and likewise also the husband hath not power ('authority') of his own body, but the wife." Husbands and wives have "authority" over their spouse's body. They "own" the other. This does not mean that the husband may say, "You have a debt to be paid." The Christian viewpoint in everything is different. Each spouse must ask, "What do I owe my spouse?"
If Christian couples do not live in this way, Satan will tempt them. This is the teaching of I Corinthians 7 in verse 5. When a husband does not give himself to his wife, or the wife to her husband, the devil grabs that opportunity to tempt the other, and lead them to be unfaithful. Then the fault belongs as much to the one who withheld himself as to the one who was unfaithful. Jesus confirms this when He said, in Matthew 5, that the one who puts away his wife for unbiblical grounds, causes her to commit adultery.
Sex in marriage is not only necessary; it is BLESSED. We do not stop with saying that it is necessary, as though that's the only reason couples give themselves to each other. Hebrews 13:4 says, "Marriage is honorable in all, and the bed undefiled." All by itself, sex in marriage is a blessed, sanctified gift of God.
Genisis 2:24 shows that sex is not the result of sin, but was blessed by God before the fall. "God blessed them and God said unto them, Be fruitful and multiply, and replenish the earth." They had no shame, as verse 25 shows: "And they were both naked, the man and his wife, and they were not ashamed." For us, the grace of Jesus Christ restores us to the right attitude and the right use of this gift of God. The Song of Solomon is unashamed in its description of this gift (chapter 1:3). Or, read how a wise father describes the blessedness of married life in Proverbs 5:15-19.
However, sex is blessed only when we keep in mind the Christian view of it. In the thinking of non-Christians, the marriage partner is only a tool for self-gratification. The Christian perspective is different, because it flows out of a Christian definition of love. The Christian does not ask, "How can my wife please me?" He asks, "How can I please my wife?" Not: "What does my husband owe me?" but: "What do I owe my husband?" This is the uniquely Christian perspective of love that gives instead of takes.
If this is the Christian perspective, then Christian parents ought to be ready and willing, yes, even jealous, to teach this to their children. Are parents concerned only to warn their children about the sin of misuse of this gift, but never to teach its good and blessed use in marriage? This is wrong and will result in warped thinking and undue distress in the children's marriages.
How and where should they be taught? Not in school. This subject is too sacred, too private, too important to be taught in public where the children are all herded into a room and the boys snicker behind their hands out of embarrassment. This is the parent's duty and privilege. If parents, instead, let television and the movie theater educate their children about this gift, it will ruin them for their future marriages. (This is one, main reason why the Protestant Reformed Churches still call their members to avoid most television programming. It is obedience to God's calling to us to be holy and to "flee fornication" - I Corinthians 6.) In addition to the warnings, parents have a responsibility toward their children to teach them this part of good family life, too. Good parents want to do this. Good parents are not ashamed of this.
But sex isn't everything. It isn't the main thing. It's the icing of the cake, if we may put it that way. Young people (and married adults) ought to be reminded of that, too. A cake with only frosting is no cake at all.
The main thing is the spiritual union between the husband and wife -- their love for God, their love for each other in Christ, their commitment to good, solid family life under Jesus Christ. This must be worked on -- by prayer, by family worship, by good fellowship with other Christians, by dealing carefully with sin and sin's influence in our lives.
Because the spiritual union can be severed by the physical, married couples ought to be counseled to have a Bible by their bedside opened to the Song of Solomon, for bed-time devotions. We must be so spiritually minded, that when we think of marriage we think of Christ and the church; for our love for our spouse -- our physical love -- reflects our passionate love for Jesus Christ and His powerful, saving love for us.
Remembering this, sex in marriage will be sanctified, truly pleasurable, and GOD-GLORIFYING.