The Greek word porneia and the Aramaic zanayeh mean more than just two unmarried persons copulating. The Greek term literally means someone who gives or receives something sold by prostitutes, and the Aramaic word literally means anyone being or using a prostitute, but its root word is also used to create words or adultery and acting promiscuously, so is understood as meaning any sex outside of marriage.
Therefore, both words cover all types of sexual acts that might be sold by prostitutes between unmarried individuals (male or female), even without an exchange of money.
The problem with the English word fornication is that it traditionally implies just copulation between unmarried persons, but this is not the limit of the Greek and Aramaic words. So this translation uses the broader term sexual immorality to convey the full range of meaning.
For more information, please see the commentary Christian Morality.