I have, until recently, supposed that fornication refers to casual sex, between people who aren't married to each other, or to anyone else, whether they are living together, or engage in "one-night stands." In fact, the Wikipedia backs that up: "Fornication typically refers to consensual sexual intercourse between two people not married to each other." I also supposed that the Bible forbids such behavior. That is a common view among Christians.
The Blueletter Bible gives an Outline of Biblical Usage for μοιχεύω, moicheuō, the Greek word translated as adultery, as follows:
1) to commit adultery
Uses of the word, fornication, in the Bible
But things aren't that simple for fornication. Fornication has a wider Biblical usage, in those Bible translations that use it, than the one given above.
A look at the list of occurrences of fornication in the King James shows that, of the 30-plus occurrences of the word, several do not refer to sexual activity at all, but to idol worship. In fact, the four uses in the Old Testament are all about idol worship, not sexual activity.
The Blueletter Bible also gives an Outline of Biblical Usage for πορνεία, porneia, the Greek work translated as fornication in the King James and other versions:
In other words, the meaning of fornication, in some versions of the Bible, is much broader than the common current definition. Like "charity" in 1 Corinthians 13, the meaning has changed. It's no wonder that several versions of the Bible don't use it, or use it less. The New King James has about half as many occurrences as the King James. The New International Version, the New Living Translation, and the English Standard Version don't use the word at all. The New American Standard Bible uses it only four times, all in the New Testament.
The Bible definitely forbids adultery, which is when a married person has sexual relations with someone who is not their marriage partner. See here for the Ten Commandments, which forbid adultery. It also condemns homosexual activity, rape, seduction, prostitution and patronizing prostitutes, as well as having sex with animals and close relatives or in-laws. (Except for the first one, the links in the previous sentence do not reference all the Biblical statements on the particular subject.) However, to my amazement, I can not find an explicit statement forbidding living with someone, in a heterosexual relationship, without being married to that person, except that Jesus did point out that the woman at the well was living with a man, but was not married to him. I also can not find a statement forbidding casual one-time consensual, non-commercial sex, unless it's the statement on seduction. Here's a good article on sexual immorality in the Bible, and there's no such situation mentioned. If you find Biblical statements on these, which don't use the word fornication, please let me know. Comments may be made below, or, if you know my e-mail address, or how to contact me personally, please e-mail me or tell me. Thanks.
Does this mean that casual sex, or cohabitation without marriage, is OK? No. One possible reason why these sorts of behavior may not be spoken of is that the opportunity may have been rare. There are statements condemning sex with someone who is engaged to someone else in the Old Testament. As indicated in the story of Mary and Joseph, engagement (KJV "espoused to") seems to have been a more serious matter than it is in most of present-day North America. Most likely, young people were engaged earlier than most couples are engaged today, and, therefore, sex with another single person may have usually been covered under the prohibition against having sex with someone who was engaged. There may have been few, or no, non-engaged persons to have sexual relations with. A second reason may be that not having sexual relations of the kind at the beginning of this paragraph isn't specifically prohibited would have been understood. There are things that everybody knew in Bible times that we don't know. To take a trivial example, most pictures and movies that show Jesus show Him with a beard. But the Bible says nothing about that. Most likely, every male wore a beard in those days, and there was no reason to explain it. When the best modern translations tell us to avoid sexual immorality, in Acts 15, most likely casual sex and cohabitation were part of what was understood to be included in that prohibition. We should have that understanding today.
There's another, more important reason why casual sex, or cohabitation, aren't explicitly condemned. That reason is that God's best plan for relationships is for a woman and a man to be married to each other. This is presented, in various ways, throughout the entire Bible, from Adam and Eve in Genesis through the church as the Bride of Christ in Revelation, and often in between. Some of the presentations in the Bible are of relationships that have gone wrong, and are presented as such, such as the stories of David and Bathsheba, of Hosea and his unnamed wife, or the picture of Israel as God's faithless wife in Ezekiel 16. Some, like the relationship between Mary and Joseph, or between Priscilla and Aquila, are presentations of relationships that have measured up to God's plan. God's plan, heterosexual monogamy, presented in various ways, some most dramatic, argues against various perversions by showing what the way ought to be.
Thanks for reading!