I don't think a person needs to be married in order to understand how it would feel to come home and find their wife in bed with another man. The pain of adultery is something we can all understand. But what causes us to feel that way? There's the betrayal of trust, for sure. But there's something that goes deeper than that. It's a feeling that we all have “built into us” - a feeling which God has given us. We can all identify with it, whether we can rationalize it or not. That feeling is jealousy.
Now, when I met my wife, neither of us were yet Christians and neither of us were virgins. This is most typical in this day and age, and yet it caused me much grief to hear that she had been with another man. Personally, I can hardly bear the thought. Not only that, but it also grieves me that I had been with another woman before I met my wife. When I had met my wife, I was foolish enough to show her some old video which happened to have my ex-girlfriend on it, boasting of how she’d “had me”. My wife, watching that, left the room holding back tears. Those feelings of jealousy are very real even in this situation, whether we like it or not. It is the same kind of hurt as when a husband or wife commits adultery. When we find our spouse has committed adultery, we are inclined to divorce them. In times past, when someone found out that their fiance was not a virgin, people were inclined to call off the wedding. These days we just grin and bear that hurt and try to minimize it. We say that we ought not to feel this jealousy, and actually train ourselves not to, because we want to protect ourselves against what is probably inevitable. But in fact we all have this common feeling of jealousy which tells us that sex outside of marriage is wrong.
The metaphor of marriage is used throughout Scripture to describe the relationship that God desires to have with each of us. We might look particularly at a passage like Ephesians 5...
In the same way husbands should love their wives as their own bodies. He who loves his wife loves himself. For no one ever hated his own flesh, but nourishes and cherishes it, just as Christ does the church, because we are members of his body. “Therefore a man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh.” This mystery is profound, and I am saying that it refers to Christ and the church. (Ephesians 5:28-32)
This passage is telling us that we should love our wives as we love ourselves. The kind of care and concern we take for ourselves we should also give to our wives; like being “one flesh”. And then Paul tells us that this kind of marriage, (the kind that God wants us to be in), is illustrative of Christ and the Church. Christ cares for us in the same way, as though we were “one flesh” with Him.
Another common metaphor in the Bible is that of adultery. Often when the Bible says that Israel “committed adultery”, it is speaking of their worship of idols. You can begin to see how this all fits together. The Biblical ideal of monogamous, faithful marriage is like a divine teaching aid for us, showing us what God's relationship with mankind is to be like. The Bible calls God a “jealous God.” Much like a husband is jealous for his wife if she commits adultery, God is jealous for His people when they worship other false gods. It breaks His heart – not that the false gods are anything, but that we should desire them instead of God. This causes God real pain, and this kind of jealousy over one you love is precisely what we should expect from a God who loves us with perfect love.
When I look back over the years of my life before I became a Christian, I really do grieve over my own sinfulness. And that grief is primarily over how that sin offended God. It sorrows me even though I did those things without really knowing or understanding my foolishness, and though I did not yet know God. It is precisely as we described before; though we hadn't yet met, my wife and I both regretted having been with other people before we met, and we both felt jealousy over it. Just as I wish I could tell those unsaved friends of mine to stop sinning because one day they might know God and really regret their present sinfulness, we shouldn't engage in sex before marriage lest one day we finally get married and have a similar regret.
If we, as a society, distort God's illustration of marriage which illustrates the relationship God desires to have with each of us, then it becomes so much harder for us to understand the love of God, and how we should respond to that love. Probably many people reading this are having difficulty understanding how a marriage relationship is like having a relationship with God, precisely because our society has distorted marriage already. We often say things like, “But I love my girlfriend – we're as good as married, we just haven't walked down the aisle together.” But this kind of relationship still distorts the message that God is giving us through the metaphor of marriage. If we had the time, we could talk about the Biblical idea of a Covenant, and explore the depths of how a Biblical Covenant, like a wedding ceremony with its vows, also open up our understanding of who God is and how He desires for us to relate to Him. In short, God is wiser than we are, and if He says “sex is for marriage only”, we ought to listen to Him and obey. It is as we live by faith, according to His commandments, that we are then able to identify and understand more and more what God is trying to show us.
And in that day, declares the LORD, you will call me My Husband, and no longer will you call me My Baal. (Hosea 2:16)