A look at lexicons like BAGD and LSJ gives πορνεία the general meaning of "prohibited sexual relations". As you say, this is fairly generic, but in Paul's case I think we can be more specific. The NT documents and early church for the most part took the Old Testament's laws on sexual relations as determining what was and was not permissible, even if they didn't necessarily adopt the OT Law wholesale or its penalties for disobedience. In this regard, it is reasonable to assume that Paul understands sexual relations outside a male-female marriage relationship to be 'unlawful' and so fall under the catch-all category of πορνεία. This is the standard reading you will find in most commentaries on the NT letters. It is unlikely that Paul adopts the prevailing Graeco-Roman norms of sexual behaviour, although he is certainly aware of them and is writing in that context. For example, in the letter of 1 Corinthians Paul is well aware that one aspect of Gentile sexual ethics is about "who owns who determines what may or may not be done." In response Paul asserts that believers belong to the Lord, and so this question of ownership-identity is redefined in theological, not merely sociological terms.
As for the second part of your question, I dare say it is almost too broad to answer. Although this standard "sexual relations beyond marital ones are prohibited" is a good place to start. You can see that basic definition modified, for instance, in the Roman Catholic tradition, where sexual acts within marriage must be open to the possibility of procreation.
Are there specific denominations or sexual acts you would like to discuss in terms of historical perspective? I'm happy to take the discussion further in one direction if you want to be more specific.
In the early Medieval church sin became slowly more defined, especially when it came to sexual issues. Repenting became a key notion as Christianity spread, developing from a terminal act to something that can be repeated several times in a person's life. Monastic orders were the first to outline what was immoral in texts known as penitentials. These initially focused on foods and superstitions (namely issues affecting those living in closed orders), but this slowly moved into the area of sexual morality - both for members of holy orders and the laity.
These Christian texts deal with sex outside of marriage, homosexuality, masturbation, rape, incest and other issues. They tackle how to best repent for these sins, balancing the religious and social cost of these acts. For example, the Old English Penitential states "If anyone deprive another of his daughter by force, he is to make amends to the relative and each of them is to fast 1 year on Wednesdays and Fridays on bread and water and on the other days partake of their food except for meat, and afterwards he is to take her in lawful marriage if the relative wishes it."
Obviously in this period we are looking at a Christian church, the Western Church, that went on to become the Catholic. These penitentials were also more common among members of the Irish tradition in the early Medieval period, being spread with the Irish missionary work on the continent.
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