Fornication is sex outside of marriage. Cheating on your wife with another woman is fornication and adultery, having sex with a girlfriend or a prostitute is fornication. If we should be perfect then we should have desires only towards our spouse, but so many people in this age have no regard for that, including Christians and "no sex before marriage" seems so ridiculously outdated.
The thing is, it is not outdated but it has been ridiculed by the world because the world doesn't seek perfection, it seeks pleasure and excitement even unto death. And many Christians don't read or study the bible and are ignorant to these teachings.
Since I started studying the bible I realized that it is one of the commandments for all believers to not fornicate, to not be in relationships that are self-serving and meaningless. To fear God rather than to love the flesh.
Proverbs 7:27 Her house is the way to hell, descending to the chambers of death.