I have not talked to you in a while, but here is my question:
You once told me that it was not sinful that sexual thoughts popped inside my head, that it was normal, but if I entertained the thought or if I picture or fantasize about being in a sexual relation with someone, then it would be like if I was committing fornication or adultery in my heart. You also told me that it is also sinful when some people masturbate while picturing someone or something they have seen in pornography.
So my question is that if you picture an imaginary woman who does not exist, and you have a sexual thought toward someone in your imagination that does not exist in real life, is that still a sin? Is that still committing fornication in the heart, since that is nothing but an imagination in your head?
"For this is the will of God, your sanctification: that you should abstain from sexual immorality; that each of you should know how to possess his own vessel in sanctification and honor, not in passion of lust, like the Gentiles who do not know God; that no one should take advantage of and defraud his brother in this matter, because the Lord is the avenger of all such, as we also forewarned you and testified. For God did not call us to uncleanness, but in holiness" (I Thessalonians 4:3-7).
One could extend your argument. Since pornography often involves touched up images, you could say that the people in pornographic images are imaginary. Does that make pornography right?
What is missing is why pornography is wrong. It is wrong because you are imagining yourself having sex with someone to whom you are not married. That constant exposure to seeing yourself, in your mind's eye, putting your penis where it doesn't belong is eventually going to corrupt your attitude toward fornication. It will weaken your resolve not to sin, and when you find yourself in a sexually charged situation you are more likely to drop your pants without thinking about the consequences. "For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, thefts, covetousness, wickedness, deceit, lewdness, an evil eye, blasphemy, pride, foolishness. All these evil things come from within and defile a man" (Mark 7:21-23). That the person you are doing in your head is imaginary isn't the problem, it is the pattern of behavior that you see yourself doing and accepting.
If you must imagine something, think about what your wedding night will be like. Don't consume your mind with sinful thoughts. But even here there can be a danger. Reality is rarely as "perfect" as our imagination. Spend too much time imagining how things ought to be and you will be disappointed when you find out how things actually are.