New Defender's Study Bible Notes
6:18 fornication. In Biblical usage, “fornication” can mean any sexual congress outside monogamous marriage. It thus includes not only premarital sex, but also adultery, homosexual acts, incest, remarriage after un-Biblical divorce, and sexual acts with animals, all of which are explicitly forbidden in the law as given through Moses (e.g., Leviticus 20:10-21). Christ expanded the prohibition against adultery to include even sexual lusting (Matthew 5:28).
6:19 your body. This is the classic verse which teaches that a Christian’s body belongs to God, not to himself or herself (modern “pro-choice” feminists notwithstanding). Since our “bodies are the members of Christ” (I Corinthians 6:15), we have no right to unite them with some other body in any sexual relation outside of monogamous marriage. Such becomes a sin “against his own body” (I Corinthians 6:18), which could easily result in one of many sex-transmitted diseases (only the worst of which is AIDS), not to mention psychological disorders.
6:20 bought with a price. The purchase price of our bodies was the infinitely precious shed blood of Christ (I Corinthians 6:19-20). Realization of this fact provides another very potent principle for discerning the rightness or wrongness of a certain behavior. Does it, or does it not, glorify God in our body and spirit?