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, 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 copulate 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.
Marital Duties
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 copulation couples have with each other.
Let marriage be held honorable by all, and the marital union be undefiled; for God will judge fornicators and adulterers. (Hebrews 13:4, HBFV).
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.
Let the husband render his conjugal dues to his wife, and in the same way also the wife to her husband.
The wife does not have power over her own body, but the husband; and in the same way also, the husband does not have power over his own body, but the wife.
Do not deprive one another of conjugal dues, except it be by consent for a time, so that you may devote yourselves to fasting and prayer; and then come together again as one, so that Satan will not tempt you through your lack of selfcontrol. (1Corinthians 7:3 - 5, HBFV).
Paul, in his letter to the church at Ephesus, 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 sexual relations 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, and 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 (which are acts of adultery - see Leviticus 18:6 - 18, 20:10, Deuteronomy 22:20 - 22, etc.).