These are the subjects that most people do not understand.
Remember that we are a shadow, as far as our existence in this world, goes.
Reality exists in Heaven only and we are simply a shadow, so to speak.
God is light and that is the image of truth and reality.
We are darkness, or something that lacks a relationship with light.
We have these two examples in this life to help us understand our true relationship with God.
Adultery is the act of a married person, engaging in a relationship with someone who is not their spouse.
Fornication is the act of an unmarried person engaging in a relationship with someone else.
In the spiritual realm, which is the reality of life, our relationship can only exist with God.
As an unbeliever, or someone who has not believed in Christ and therefore making you a non-Christian, then your relationship with creation, your interactions with the world etc., make you a fornicator. The world is not your spouse and never will be.
When you believe in Christ, then you become a believer, or a Christian. You become a member of Gods family and therefore have a genuine and truthful relationship with God.
Under that relationship with God through Christ (you had to believe in Christ in order to become a member of Gods family), then you are bound by the rules of the marriage relationship.
Those rules are simply, to confess your sins, and thus remain within a functional relationship (in fellowship) with God.
God the Father is generally designated as the target of our relationship, God the Son (Jesus Christ) is the means of that relationship, and God the Holy Spirit is the environment (fellowship) or the power of that relationship.
When you violate any of these rules, ie. sin and are thus out of fellowship, ignore your daily study and thus are not functioning or not advancing in your spiritual life, then you are in effect attempting to return to the state of the unbeliever and have a relationship with things that are not of God, namely with the world.
You are in effect imitating the lifestyle of the unbeliever.
You cannot become an unbeliever again. You cannot unbelieve once you have believed, but you can turn your back on the responsibilities and obligations of the spiritual life, thus violating the rules of your relationship with God.
Adultery describes the Christian who has a dysfunctional spiritual life.
Fornication describes the unbeliever, who has no spiritual life