We all know the Scripture in Hebrews:
Hebrews 13:4 - The marriage bed is to be undefiled; for fornicators and adulterers God will judge.
It starts with, "Marriage is to be held in honor among all..."
Fornication defiles the wedding bed. It contaminates the union between man and wife.
There are many analogies that show God's Love for us. Jesus spoke of us being sheep, and He is the shepherd - showing that he leads and watches over us.
He too is Lamb of God - showing that He is not demeaning us by calling us sheep. There are analogies of farming that show the care and patience and hard work needed. We see so many harvest symbols in the Bible, there is even a Feast Day schedule set up around this. But of all the various analogies that could have been chosen to represent the union of Jesus and the Church, marriage and only marriage is given. Husbands are told to love their wives even as Christ loves us. Paul speaks a the mystery contained in Genesis when the Scripture mentions leaving father and mother and cleaving to ones' spouse. No other analogy is given this high honor, used to show the very union of God and man, and we are commanded to keep the marriage bed holy, hold it in honor for this reason.
Interesting to me that fornication (not just adultery) is listed as something that defiles the marriage bed. But when you think about it, of course it does. People who cheat on themselves may be willing to forgive themselves, who wouldn't? Everybody has love to self - that's our selfish nature, but that doesn't mean that we don't incur consequence. When we cheat on ourselves in this manner, the bible says that we are taking sin into our very members. That's not a normal thought these days, but sexual sin is set apart in this manner and we are told to run from it for this reason. It's insidious.