One of the mysteries of marriage is that two become one. This is God's design, one that He instituted back in the Garden of Eden, at the start of human history, with Adam and Eve: "Therefore a man shall leave his father and mother and be joined to his wife, and they shall become one flesh." Genesis 2:24.
A husband and wife become one in the act of marriage, that is to say, through sexual intercourse. Before their wedding night, they are two individuals. After their wedding night, they are one flesh. Two complete and separate individuals unite to become one in a permanent, unbreakable, life-long union of God's design. We see the fruit of this unique union in children. A child is a whole and separate individual created by the union of two others. Two create one.
When a man and a woman come together in marriage, they bring their sexual needs into the marriage with them. Both the wife and the husband are entitled to have sex, but the person who is to fulfill those sexual desires is the opposite spouse—no one else. This rules out sex with people outside the marriage (adultery), and sex with oneself (masturbation).
In the book of First Corinthians, Paul tells you and me: "Let the husband render to his wife the affection due her, and likewise also the wife to her husband."
"Let the husband render to his wife the affection due her" means the husband should "fulfill his marital duty to his wife." His marital duty is to have sex with his wife when she wants it. And the wife is to do the same. She is to have sex with her husband when he wants it. Both are to do so with mutual respect and consent.
Notice the language of Scripture here. Sex is something that is "due" the wife. She is entitled to have sex with her husband. And the husband has a "marital duty," a responsibility within the marriage, to meet the sexual needs of his wife. When the wife has sexual needs, she is not to satisfy them on her own through masturbation. When the husband has sexual needs, he is not to satisfy them on his own by pleasuring himself. Sex is what the other spouse is for.
Sex is literally designed to take place between a male and a female, between a husband and a wife. Sex requires two people joining as one. What this means in daily life is that neither a husband nor a wife are to take matters "into their own hands" where sex is concerned. If they want to have sex, they are to turn to their spouse. Not someone else. And not themself.
