Although children are a natural result of sexual intercourse between a husband and wife and are called "a gift of the LORD" (Psalm 127:3, New American Standard Bible), the Bible does not prohibit sexual relations within marriage purely for pleasure. Indeed, it encourages such union.
The idea that sex was dirty and evil was an idea that crept into Christianity from early Catholic teachers. Their compromise with the obvious reality that sexual activity was necessary to have children resulted in their teaching that sex should only be engaged in by married couples when they wanted to have children. Yet there is no such instruction in the Bible.
Genesis 2:24 says, "Therefore a man shall leave his father and mother and be joined to his wife, and they shall become one flesh [have sex]." Proverbs 5:15-19 encourages couples to enjoy sexual pleasure together within marriage. Paul says that husbands and wives should render the sexual affection due each other—refraining only during times set aside for prayer and fasting (1 Corinthians 7:3-5).
No passage in the Bible forbids husbands and wives from having sex for pleasure when not trying to conceive children. There is nothing wrong with couples using contraceptive methods, provided they are not physically harmful, to postpone having children until the time of their choosing.