"Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.
What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own?"
1 Corinthians 6:18-19
I think it is hard for us to see how we are hurting ourselves sometimes. With some things it is more clear... cutting yourself is obviously harmful. Smoking and drinking less obviously so, but lung cancer and vehicular homicide are pretty bad, and we can usually connect the dots and avoid the risk. Other actions have less evidence of immediate harm. With fornication, we think of the risk of pregnancy or the heartbreak of a broken intimate relationship, and those things are big consequences, for sure... but are they as bad as cancer or slicing ourselves? We usually think not... that we're doing nothing bad to our bodies, and perhaps we even think that we are doing them good.
I think to understand what Paul (and God) are getting at here we have to think of fornication in comparison to something else that can be righteous and good... that is not inherently evil, as sex is not. It's more like a food addiction perhaps. It isn't bad to eat. :) Eating sustains life. But when we become overly obsessed with the process, we can harm ourselves... binging and purging, or teaching our bodies that eating is the way to solve our problems or fill our emotional needs. Instead of eating to take care of our bodies, food can become the central focus of our existence, and we harm ourselves and others by obsessing over it, making it the focus of all of our social interaction, and dreaming about it or looking at pictures of it even when we are not eating. We harm ourselves by not teaching our bodies limits and connecting all kinds of emotions and thoughts and our whole lives sometimes to this one thing, which should have a place, but not above God or our well-being. We harm ourselves by thinking of food in a different way than God has intended, and convince ourselves that it is okay, since eating isn't inherently evil, after all.
There are, of course, differences. I think that God designed sex to be attached to affection and emotion naturally, and it forges a deep connection. But when we treat it frivolously then we harm ourselves by making deep connections with people that we don't intend to be connected with at all. This breaks us, inside. The wounds aren't more than God can heal, of course, but it isn't any better than cutting ourselves.
And last of all, of course, we are not our own. When we do anything outside of God's rules, we are saying that we know better than he does. That we can make it on our own, we're happy to do the whole life thing ourselves, etc. And we forget, or ignore, that the only reason that we have any chance at all in this life is that Christ paid the price for our sins. We're placing something in our lives above him, and saying, this is more important than God... and thus it becomes an idol of sorts. It might be food, it might be sex... it might be anything. But whenever we do that, we harm ourselves immeasurably, because not only do we lose the spirit, but we teach ourselves that we can do whatever the crap we want... whatever feels good at the time. And then, later, if we want to repent or change, our bodies are like no... indulge me. And it is hard to break out of that self-taught lesson... both mentally and physically. We hurt ourselves with almost all sin by training ourselves to ignore God.
Today, let's not harm ourselves. Let's undo whatever harm we can, and make our bodies and minds more welcoming to Christ. Let's invite the spirit in and not drive it away. It's much harder to repent of things where we've trained ourselves to ignore the spirit and dismiss God from our lives and thoughts... but we *can* still do it... and God will help us change our hearts, if we are willing to ask.