What does 1 Corinthians 6:16 mean?

Paul is confronting the Christians in Corinth about the issue of sexual immorality. Some, apparently, had the idea that since sex is just a normal human appetite, like hunger for food, one ought to satisfy it as they see fit. Also, they argued, the body will eventually die and decay, so it doesn't really matter what we do with it. In this way, they reflect the pagan culture around them, which embraced such attitudes (1 Corinthians 6:12–13) and was drenched in both sexuality and idolatry.

Paul has rejected both ideas by elevating the purpose and power of both our bodies and of sex. A Christian's body will be resurrected, as Christ's was, and so it will not end at death. More than that, though, our bodies right now are, mysteriously, members of Christ. They are part of Him and part of His purpose for us.

Paul has also rejected the false idea that "sex is just sex," like any other appetite. In fact, he now declares that even sex with a prostitute, a momentary transaction, causes a person to become "one body" with her. He quotes from Genesis 2:24, which reveals God's design for both sex and marriage, a man and woman becoming united, physically and spiritually, as "one flesh" or one body.

Putting this and the previous verse together, Paul's point is that a Christian's body, as a member of—or "joined to"—Christ, should never at the same time become a member, one body, with a prostitute. This would apply to all forms of sexual immorality.