in order to please God

1 Thessalonians 4:1-8

The key phrase in this section is: "how to live in order to please God." Paul isn't overly concerned by the behaviour of the Thessalonians; in fact, he commends them for living in a way that is pleasing to God. However, he does include some reminders - in this section he reminds them about getting rid of lust.
Paul reminds the Thessalonians that God's will is their sanctification and that part of that process is sexual purity. He also reminds them that sexual immorality is not just an offense against another person but also against God and a rejection of this instruction is a rejection of God. This is consistent through the whole Bible: lust is a sin because it is an affront to God's purity, holiness and faithfulness. In other words, one way to live in order to please God is to turn from lust or flee sexual immorality. However, I must emphasize that the Bible does not teach that sexual desire is a sin - God created us as sexual beings - but that lust is.
So when does sexual desire become lust? I think the passage gives us some answers. First, desire becomes lusts when there is a lack of self-control. Paul talks in another letter about not letting his body or his desires control him but working to make sure that he is always in control of his desires. Secondly, desire becomes lust when I no longer regard the holiness of God. In other words, if my desire causes me to think or act in a way that does not reflect the character of God, then it has become lust. Thirdly, desire becomes lust when I have no regard for the honour of the other person. Each person has been created in the image of God but when I strip them of dignity in order to use that person, in my fantasies or in reality, to fulfill my own desires, I have robbed them of the honour they are due as a human.