For many people, the term carries an overtone of moral or religious disapproval, but the significance of sexual acts to which the term is applied varies between religions, societies and cultures.
The First Epistle to the Corinthians states "Flee from sexual immorality" and lists adulterers and "those who are sexually immoral"/practicing-fornicators in a list of "wrongdoers who... The Apostolic Decree of the Council of Jerusalem also includes a prohibition of fornication.
Throughout history, most theologians have argued that any and all forms of premarital sex are immoral.
and prostitution are all explicitly forbidden by name.
Paul is preaching about activities based on sexual prohibitions laid out in Leviticus in the context of achieving holiness.