ANSWER: Your question concerning sex between couples who are married is answered rather succinctly in the Bible. The Apostle Paul's first letter to the Corinthian church tackles this rather sensitive topic.
In 1Corinthians 7 he first states that both the husband and the wife in a relationship ought to render to each other "due benevolence" (the KJV translation uses this term as a euphemism for stating they owe each other sex). He then gives us the underlying principle that should govern such acts within marriage.
The wife does not have authority (unilateral and complete power) over her own body (in regard to sex), but the husband does . . . (1Corinthians 7:4, NKJV)
It is important to note that nowhere in Scripture is it commanded, or even suggested, that a married couple should only have sex when they are trying to have children. There is also no indication that any copulation occurring between two people, within the bounds of marriage, is a sin of itself.
The Apostle Paul states, in the book of Hebrews, that marriage is an honorable practice and that the marriage bed is "undefiled," meaning that there are really almost no restrictions concerning any sex couples have with each other (Hebrews 13:4).
Paul, in the Bible, calls the physical relationship or sex between a married man and woman a "marital duty" and stresses that they should not abstain from it unless they both agree to do so for a short period of time (1Corinthians 7).
In his letter to the church at Ephesus Paul stresses the role each person in a marriage should play, especially in regard to each person's duty to participate in sex. He also discusses how they should treat each other. So long as these guidelines are followed, any relationship will flourish, even a childless one.
Wives, submit yourselves to your own husbands, as to the Lord; Husbands, love your own wives, in the same way that Christ also loved the church . . .
In the same way, husbands are duty-bound (they have an obligation before God) to love their wives . . . (Ephesians 5:22, 25, 28, HBFV).
King Solomon, who was given extraordinary and one-of-a-kind wisdom by God, warns us that lust and the illicit sex it can lead to can wreak havoc on a person's life (Proverbs 6:25 - 26). The Apostle Paul also warns to flee immorality both before and during marriage (1Corinthians 6:18).
The ability to have sex is one of the gifts God built into humans upon their creation (Genesis 1:28). He gave it to use for our benefit and happiness. In the Bible He encourages couples to enjoy this type of intimacy within marriage (Proverbs 5:18 - 19, Song of Solomon 1:12 - 13, 7:6 - 13, 8:1 - 3).
What God does not approve of, however, is using the gift of sex wrongly by performing it before marriage (fornication or lust) or with someone who is not one's marriage partner (adultery - see Leviticus 18:6 - 18, 20:10, Deuteronomy 22:20 - 22, etc.).