How do we reconcile Hosea 1:2, and I Corinthians 6:15-18? Is sinning permitted if it serves a greater purpose? e.g. Robin Hood scenario, etc.
What makes something a "sin outside the body" compared to "against own body", e.g. gluttony, etc.
You appear to be assuming that Hosea sinned in marrying a prostitute. First, we must understand that God does not tell people to sin. "Let no one say when he is tempted, "I am tempted by God"; for God cannot be tempted by evil, nor does He Himself tempt anyone" (James 1:13). What God told Hosea to do was not sinful.
Paul was arguing in I Corinthians 6 against sex outside of marriage, sexual immorality or fornication. Hosea married Gomer, so the arguments don't apply in his situation. She lived a life of harlotry, but by marrying Hosea she left that life.
"Do you not know that your bodies are members of Christ? Shall I then take the members of Christ and make them members of a harlot? Certainly not! Or do you not know that he who is joined to a harlot is one body with her? For "the two," He says, "shall become one flesh." But he who is joined to the Lord is one spirit with Him. Flee sexual immorality. Every sin that a man does is outside the body, but he who commits sexual immorality sins against his own body" (I Corinthians 6:15-18).
Can sex be indiscriminately indulged in without harm? Solomon argued against it. "Whoever commits adultery with a woman lacks understanding; he who does so destroys his own soul. Wounds and dishonor he will get, and his reproach will not be wiped away" (Proverbs 6:32-33). Can I indulge in it at any time or with anyone? A woman's husband would disagree (Proverbs 6:34-35). But most importantly, can I take the body God has given me to use for a holy purpose and join it to someone living a life of sin? (I Thessalonians 4:3-5).
Paul proves that sex binds the participants. It is not a permanent or stable bond, but it is a bond nonetheless. Those who engage in sexual sins are physically coupled during the act. They become for the moment one body (the Greek word soma). This is different from sex in marriage where the two become on flesh (the Greek word sarx). Illicit sex is just a joining of bodies, which is unstable. Marital sex is the joining of two human beings into one life. And our joining to the Lord is an even greater bond, being a spiritual fellowship. Marital sex is compatible with our spiritual bond because the Lord blesses it (Hebrews 13:4). Illicit sex is not compatible with our spiritual bond with Christ.
Why should we take what God has freed from sin (Romans 6:1-7) and then bind ourselves to sinners, even on a temporary basis? We are a part of Christ (Ephesians 5:30) and when we sin, we are attempting to force sin to join with Christ through us. It won’t work because what is flawed is cast off. Our reaction to sins, such as fornication ought to be run away from it as far and as fast as we can.
So why did God have Hosea marry a former prostitute? Here is a lesson that more people need to learn. People don't change their character just because they get married. While Gomer was given an opportunity to leave her life of sin, there was no radical change on her part, and predictably the temptations of that past life drew her back into sin. God counted on that weakness to serve as an illustration of what He was going through with Israel. God didn't cause her fall; He knew she would eventually fall because marriage doesn't cause a radical change in behavior.
There is no such thing as doing evil to cause good. "And why not say, "Let us do evil that good may come"? --as we are slanderously reported and as some affirm that we say. Their condemnation is just" (Romans 3:8). Robin Hood's stealing was not justified by the evil of the rulers in his day. Being "in love" doesn't justify fornication.
Every sin causes harm. Stealing harms the person stolen from. Lying harms the person lied to. What Paul points out is that fornication may seem to be a sin against the person you had sex with, but it isn't (we are assuming consensual sex here), it is a sin against yourself. Fornication is a sin that harms the person who does it. Though each consents to it, each is harming himself at the same time.