To lust is to sin
It is clear from the word of God that fantasising falls under lust, and to lust is to commit sin. Humans that we are, we have to seek God in prayer and fasting to take away the desire to fantasise. There are no ifs and buts. When people are saved, they become different. They deny the flesh and if they cannot stand the temptation of the flesh, then they should get married so that they would not enter into temptation.
There are many passages in the Bible that address this matter, but I would stick to one. Galatians 5:19-21. "The acts of the flesh are obvious: sexual immorality, impurity and debauchery; idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God." This passage clearly talks about sexual immorality. The Bible says that those who practise these things will not enter in the Kingdom of God.
People in the world can do whatever they want to do, but fantasising is lusting. Fantasising is a natural thing, yes, but I believe we are talking about living for and serving God, which is on another spiritual level. If one is a Christian, there is a very fine line between fantasising and lusting.
Dear Miss M.,
I can see that you are a Christian and you believe that it is wrong to fantasise. I appreciate your comments, and I must say to you that everybody has had the experience of fantasising. And some people are so ashamed of what they fantasise about. They will never discuss it with their close friends. Young lovers fantasise about what it would be to make love with each other. Whether that is wrong or right, it is not for me to judge.
I thank you, nevertheless, for your comments and I pray that God will continue to bless you.