I remember as I reached my teenage years being warned about it. It was standard in lessons on resisting the temptation of premarital sex. Of course I was too young and naive to understand it all, but finally someone gave me an answer to that line that made sense: "If you loved me, you wouldn't ask me to go against my principles."
After I got married I learned a far more valuable truth about it all. It takes more love to live together day after day after day than it does to sleep together. Young people, look at your parents or even your grandparents. Think of all the storms they have weathered in their lives together. Think of the sacrifices they have made, not only for you, but for each other as well. That is love. Sex is not love. Sex is one type of glue to make that more meaningful love continue, but by itself it is nothing but two people using each other for a momentary thrill. Does that sound like love?
And as for you married folks who have become enamored by someone besides the person you made a commitment to in the presence of God, you ought to be wise enough to know this by now. If you decide to break that spiritual contract and make a new commitment, guess what? You will still see him sweaty and unshaven at times. You will still see her in cold cream and rollers. He will still belch out loud while scratching what will eventually become a pot belly, and she will still wear a ratty old housecoat and go around the house without makeup. He or she won't really be any better than the one you have now. In fact, since that person probably broke a commitment like you did, you won't even have the luxury of ultimate trust in your relationship.
Love is about real life, and real life is about giving and sacrificing and enduring, not glamor and excitement and no, not endless sex. The wise ones have learned to see beauty in the beaming wrinkles and the soft extra rolls around the middle and excitement in the smiling droopy jowls and calloused but gentle hands. If you never learn those things, you are to be pitied, not celebrated. But you won't need to worry about the lines handed to you by a seducer. Where true love lives, there are no lines, just warmth and compassion and an assurance that person will always be there, no matter what.
And this second thing you do. You cover the LORD's altar with tears, with weeping and groaning because he no longer regards the offering or accepts it with favor from your hand. But you say, “Why does he not?” Because the LORD was witness between you and the wife of your youth, to whom you have been faithless, though she is your companion and your wife by covenant. Did he not make them one, with a portion of the Spirit in their union? And what was the one God seeking? Godly offspring. So guard yourselves in your spirit, and let none of you be faithless to the wife of your youth. For the man who does not love his wife but divorces her, says the LORD, the God of Israel, covers his garment with violence, says the LORD of hosts. So guard yourselves in your spirit, and do not be faithless (Mal 2:13-16).