Sexual immorality refers to any kind of "unsanctioned sexual intercourse," which, given Paul's Jewish heritage, refers to any kind of intercourse outside marriage. Such activity could be viewed as contrary to walking in love in that sex outside the covenant of marriage does not properly respect the other person—without proper commitment, it uses and abuses the other.Pillar Commentary, Ephesians
In biblical times or in ancient Greece, sex was regarded as something that human beings did; today it is considered to be something vital to who human beings are.
To use a distinction deployed by philosopher Roger Scruton, pornography is about bodies, not faces. If sex is just about my pleasure, any body will do as a partner. But in a marriage, the specific identity of the sexual partners is critical. The purpose of sex is not to have sex but to make love, to reinforce a relationship with a particular person—or, to use Scruton's terminology, with a face, not just with a body.
If the expressive individual sees personal satisfaction or happiness as central to the fulfilled human life, then pornography allows for the realization of that in sexual terms. It presents the sexual act as something whose significance is found simply in the pleasure of the observer or consumer.
The term πορνεία more frequently refers to women's violation of the marital relationship since "adultery" (μοιχεια) was used primarily in reference to men's activity.Remarriage in Early Christianity, 143
The New Testament often distinguishes πορνεία from μοιχεία (Matt 15:19; Mark 7:21; 1 Cor 6:9; Heb 13:4). Matthew regularly uses μοιχεία and its cognate verbal forms (e.g., Matt 5:32; 19:9!) and yet chooses πορνεία for the exception clauses.Remarriage in Early Christianity, 137
As the Empire ground down, the decadent Romans were given to a thirst for violence and a gratification of the senses. This is especially evident in their rampant sexuality.How Should We Then Live, 24
can be found in Greek literature with reference to a variety of illicit sexual practices, including adultery, fornication, prostitution, and homosexuality. In the OT it occurs of any sexual practice outside marriage between a man and woman that is prohibited by Torah. This sense is retained and intensified in the NT, which "is characterized by an unconditional repudiation of all extra-marital and unnatural intercourse."Mark Pillar Commentary