'Porne', the root of the word fornication, means 'prostitute'. So does this just mean that we should stay away from red light districts? I believe there is a wider principle here and we need to ask what it is about prostitution that makes it so incompatible with having the spirit of Christ?
First of all, a prostitute's favours are not free. Whether sex or 'love' in another form, they are only given on condition they are paid for. Within this arrangement, a person must earn their acceptance by what they do or by what they give . The person being paid is treated as a commodity, not a human being in their own right. Neither is 'loved' for the person that they are, but become an object, valued only insomuch as they can provide gratification to the other party's eyes and flesh - or pocket. This is diametrically opposite from the spirit of the gospel, where God's love is unconditional, where he sees us as whole beings and loves us for the person we are. Those who have his spirit should also hold others in the same high esteem, and that includes accepting and cherishing their partner as a whole person within the physical relationship.
In prostitution an unequal relationship is set up of master and servant. This can manifest itself in a number of ways. Jesus said that we are not to be like the heathen, who dominate and lord it over others (Matthew 20:25-28). We should not use liberty for an excuse to gratify the flesh, but by love serve one another (Galatians 5:13) looking to the other's interests rather than one's own (Philippians 2:4).
Fornication is essentially, therefore, any exploitative act involving sex, whether done by the body, the eyes, or the mind. It involves a fragmentation, in the mind of the perpetrator: the partner (or even the self) in their entirety - their inner self, their unique personhood, their pricelessness - is separated from the act, the sight or the thought of anything they consider sexual. Sometimes it shows up as not regarding one's life partner as a sexual partner, and this can lead to one or both parties to the marriage turning their romantic and sexual attention elsewhere. Sometimes the totality of the self or partner is obscured by the substitution of a false persona, through behaviour or clothing or in other ways. Sometimes the person is visually or psychologically broken down into body parts (e.g. "Great pair of legs coming this way!") - again, clothing is sometimes used to suggest this, and a great deal of pornography - and indeed 'respectable' commercial advertising - is based on this approach.
Thus fornication is any sexual self-gratification at the expense of the other, or without welcoming the reality of the other, and so can occur between people who are married to each other, as well as those who are not. 'No premarital sex' may be a useful rule for those whose understanding can so far only stretch to that: it will give some protection against heartache - and, of course, unplanned pregnancies (no loving act would be at the risk of the interests of any potential child). But here we are going way beyond mere rules.
So what is the alternative?
The Song of Solomon depicts the joys and ecstasies of physical intimacy within the context of taking delight in sharing all aspects of life together and each appreciating the wonders of the other as a whole person - their physical attributes, yes, but also their achievements, personality and character.
Jesus speaks about two becoming 'one flesh' as a result of 'cleaving' - tenaciously clinging on. This suggests an intimacy that is more than merely physical.
In 1 Corinthians 7 Paul says, "to avoid fornication, let every man have his own wife, and let every woman have her own husband". Many Christian teachers stop there, because their definition of fornication is premarital sex, but Paul doesn't stop at this point because being married will not in itself prevent fornication. He goes on: "Let the husband give to the wife due benevolence: and likewise also the wife to the husband." Benevolence - wellwishing, affection, respect, consideration, kindness, compassion, tenderness. He takes it further: "The wife has not power of her own body, but the husband: and likewise also the husband has not power of his own body, but the wife. Do not defraud one another ..." In other words, they are to be available to each other: each should consider their own body belongs to the other, rather than that the other's body - and their own - belongs to them. Paul is talking about the self-giving love that we see in Jesus, spilling over into and governing the physical and sexual aspects of our relationships. Every part of our lives must demonstrate this kind of love.