There is a website (I won't link to it) that classifies itself as a Christian website for swingers. In case you're not up to terms on this concept, the "swinging" about which they're speaking is not a playground for kids nor a baseball term. It is the free exchange of spouses for sex. The website has multiple explanations about how "fornication" and "adultery", clearly and unavoidably condemned in Scripture, are not actually clearly and unavoidably condemned in Scripture.
Take, for instance, the term "fornication". As it turns out, the Greek term for this is pornea (or some derivative thereof) and references all sexual sin. It's ... vague. In the Old Testament, the problem was not sex but the change in the value of the bride. And adultery is an outdated problem based on the ownership principle of wives, so it's no longer an issue. Bingo! We're all clear! (Another argument is that adultery is only idolatry. This, of course, only works if you don't read your Bible. Adultery may serve biblically as an illustration of idolatry, but the two are not one thing.)
It's odd, then, to read this:
Let marriage be held in honor among all, and let the marriage bed be undefiled; for fornicators and adulterers God will judge (Heb 13:4).There are, in this passage, two types of people. There are those who honor marriage and those who don't. How is marriage honored in this passage? There are those who "let the marriage bed be undefiled" and those who don't. End of types. No others.
Within the "those who don't" category there are two subcategories referenced by the author of Hebrews. In this category there are "fornicators" and "adulterers". Thus, honoring marriage in this context is a product of keeping the marriage bed undefiled, and defiling the marriage bed can either be by fornication or adultery.
Now we have a basis for understanding pornea as something other than adultery. That is, in its original form, pornea is broad. It references all sexual immorality. That would include prostitution, pederasty, adultery, bestiality, incest -- all types. But the author here differentiates and sets "adultery" in a class by itself. Thus, fornication would refer to all sexual relations of those who are not married and sexual relations with someone outside of your spouse would be adultery. The conclusion is also unavoidable. "Marriage bed" is moral sex worthy of honor and all other, either before or after marriage, is not.
The Bible is clear. Marriage must be held in honor. The easiest place to see this is in the marriage bed. Keep that undefiled. That means that all forms of sexual relations apart from that expressed between husband and wife are classified biblically as sin. All forms. All sexual relations apart from that of a married couple are an assault on marriage. Now do you begin to see that marriage is under attack today? Are you part of that attack?