The New Testment contains a Greek word that describes outlawed sexual behavior. The word is "porneia" which is commonly rendered as "fornication" or "sexual immorality" in modern English Bible translations. According to the Miriam-Webster dictionary, fornication means "consensual sexual intercourse between two persons not married to each other." The problem is that porneia is a broad, general term that means sexual behavior condemned by God in His law, which is spelled out in detail in the Holiness Code of Leviticus. Porneia includes, but is not limited to, incest, rape, and adultery. Obviously "fornication" is a bad translation, since it is limited to "consensual" behavior. Furthermore, there are examples of people in the Old Testament where two people not married to each other had sex and it was not considered sin. Many godly men of God had concubines, for example, and the law in Exodus had provision for a man to have sex with a slave or servant.
The other term commonly used, "sexual immoratity" (NIV), is a better choice for the English but still leaves open the question of what it entails. It is rarely, if ever, used as the general term it should mean: "illicit sexual behavior as defined by the Torah". It is usually used as a term to define anything the hearer believes is immoral sexually, so polygamy, concubinage, singles sexuality, viewing pornagraphy (another term that needs defining) all forms of homosexuality, and even masturbation is usually included in the concept. Again the problem is that the way "porneia" is used in the NT and in the OT Greek translation (Septuagint), it refers to a set of well-defined outlawed sexual behaviors. To see what it actually means, one must go back to its roots. Porneia in the OT was assocated with the a long list of specific sins found in Leviticus like adultery, bestiality, incest (very detailed on what constitutes it), idolatry-related sexual behavior, and rape.
In the NT, the best way to determine what it means is to look at the places where it is mentioned that includes examples. (When the word is used in a list, it doesn't provide a clue of its meaning). There are only four that I have found and they describe incest (where Paul says "a man has his father's wife"), adultery (where Paul says don't defraud a brother), sexual idolatry (where Paul cites a story from Numbers that is clearly about idolatry), and prostitution (where Pauls says don't unite Christ in you with a harlot).
So, why does the church associate other things with sexual immorality beyond these such as singles sexuality, polygamy, masturbation, pornagraphy, oral sex, and other behaviors?