ANSWER: Your question concerning sex is answered rather succinctly in the Bible in Paul's first letter to the Corinthian church.
3 Let the husband render to his wife the affection (the KJV has 'due benevolence,' which is a euphemism for stating the husband owes his wife sex) due her, and likewise also the wife to her husband (provide this conjugal relationship to her husband). 4 The wife does not have authority over her own body, but the husband does. And likewise the husband does not have authority over his own body . . . (1Corinthians 7:3 - 5, New King James Version Bible)
There is nowhere in the Bible a command or even a suggestion that a married couple should only have sex when they are trying to have children. There is also no indication that any copulation occurring with two people, within the bounds of marriage, is a sin of itself. In fact, 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 how couples may perform in bed.
Paul calls the physical relationships between a married man and woman a "marital duty" and stresses that the husband and wife should not "deprive each other except by mutual consent."
In his letter to the church at Ephesus, Paul stresses the role each person in a marriage should play. He also discusses how they should treat each other. So long as these guidelines are followed, any relationship will flourish, even a childless one.
22. Wives, submit yourselves to your own husbands, as to the Lord; 25. Husbands, love your own wives, in the same way that Christ also loved the church . . . 28. In the same way, husbands are duty-bound (they have an obligation before God) to love their wives . . . (Ephesians 5:22, 25, 28)
As a warning King Solomon, who was given extraordinary wisdom by God, warns that lust and the illicit sex it can lead to can wreak havoc on a person's life. The Apostle Paul also warns to flee immorality both before and during marriage.
25. Do not lust after her beauty in your heart; nor let her seduce you with her eyelids 26. For by means of a harlot a man is vanquished to a piece of bread; and the adulteress will hunt for the precious life. 27. Can a man take fire to his bosom, and his clothes not be burned? 28. Can one walk upon hot coals, and his feet not be burned? (Proverbs 6:25 - 28, Holy Bible - a Faithful Version)
18. Flee sexual immorality. Every sin that a man may commit is outside the body, but the one who commits sexual immorality is sinning against his own body (1Corinthians 6:18, HBFV)
According to what the Bible says, God gave humans the ability to have sex within a marriage for our benefit and happiness (Genesis 1:28, 2:18). It is therefore not sinful to have it within the confines of a marital relationship.