Paul puts an interesting twist on the argument for sexual purity. It’s obvious that if we are sexually immoral, we are sinning, but Paul points out that we are robbing others also - the person we are immoral with and that person’s current or future spouse.
A point Paul must not have covered when he was in Thessalonica was what happens to Christians who are already dead when Christ returns. Paul assures them they will rise from the dead to be with Jesus. This is why a funeral for a Christian does not have to be a totally sad event. We know we will miss our loved one, but we understand they are with Jesus.