Recently, I preached on Hebrews 13:4:
Let marriage be held in honor among all, and let the marriage bed be undefiled, for God will judge the sexually immoral and adulterous.
How do we keep the marriage bed undefiled? What should a Christian sexual relationship look like? When we ask this question we tend to begin with the sins. Is this a sin? Is that a sin? But that complicates things. The truth is simple to understand, but hard to live out due to our wayward hearts. The only proper sexual expression is between one and one woman within marriage. Every other form of sexual expression is a sin. Fornication, adultery, bestiality, rape, masturbation, sodomy, lesbianism, multiple wives, and pornography are all sins by the standard that is set in Genesis 1-2. One man and one woman in a marriage covenant until one is dead is the boundary for Christian sexual activity.
This does not mean that all of these sins are equal in their sinfulness. Is fornication as bad as adultery? No. Is adultery as bad as sodomy? No. Does this mean divorce is always wrong? No. Does this mean it is impossible to sin against your spouse sexually and yet remain sexually faithful? No. A spouse can never sleep around or look at porn and still be sinning in the marriage bed. And yet despite all the “but what abouts” the Lord has given us a paradigm in Genesis 1-2. We love to find loopholes. What is allowed? What can I get away with? Is it really that a big a deal if I masturbate, watch a little porn, or flirt with the married man at work? However, our goal should not be to see how much we can get away with. Instead we should be striving for and teaching the standard the Lord set up: one man and one woman married to one another and faithful to one another in heart, mind, and body until death separates them. The question should not be how close is this to the line, but rather how close does my life conform to the Biblical standard.