Something I haven't seen said here yet is this. Marriage is intended to be a picture or figure of Christ's relationship with the Church. The Church is the Bride of Christ, and her role is the same as the wife's role in Scripture to her husband, and vice versa.
Christ, the Husband, leads, provides, protects, directs, and has spiritual authority or headship.
The Church, the Bride, loves, serves, honors, obeys, and submits.
Just as marriage is a picture of Christ and His Bride, so fornication is a picture of heathen idolatry and self satisfaction.
There is also the "one flesh" aspect which some of you have mentioned. It's a mystery how exactly this works but man and wife become one organism together. "What God has joined let no man put asunder."
I heard - in my youth - an evangelist one time say that nothing stands between a person and God like sexual sin. This is because all sexual sin is a perversion of spiritual reality, a broken and lying image, if you will.
Fornication - on a purely practical level - is destructive to the nuclear family and a future marriage as well. Why? Well, if someone has a few dozen partners prior to marriage, they might truly love their spouse but they may have memories of better sex with someone else. Comparing and envying leads to dissatisfaction. If you save yourself for marriage, sex within the boundaries of marriage will be a unique experience that will be incomparable to any other experience. The more you play around before marriage, the more you are dulling the pleasures of marriage later on.
Not to mention the issues with exes that may suddenly become available again down the road, leading to sexual temptation, etc. etc.
