"You shall not commit adultery" (Exodus 20:14) is not limited to sex outside of marriage. God commands us to abstain from all sexual immorality, including premarital sex (fornication), homosexuality, incest, voyeurism, bestiality, public nudity, and much more (see Leviticus 18 and 20). In the New Testament, Paul adds licentiousness, uncleanness, and lewdness (Ephesians 4:19), which largely deal with liberal attitudes towards sexual matters.
Men seem to "get away" with adultery while unfaithful women are considered tramps. This double standard is ancient. In John 8:3-11, a crowd was ready to stone a woman caught "in the very act," but where was the adulterous man? God's Word, however, deals with both sexes equally.
It does not take sexual sin lightly either. Leviticus 20:10 commands death for both participants for adultery, as well as for sodomy and homosexuality (verse 13). Paul reminds us that God killed twenty-three thousand Israelites in one day for sex sins (I Corinthians 10:8). In Abraham's day God destroyed five entire cities by fire for their aberrant sex practices. God sent the Israelites and Jews into captivity for immorality, among other reasons.
Jesus struck the heart of the matter in His Sermon on the Mount. Christians must not even lust after another—fantasizing or committing sex with them mentally (Matthew 5:27-28). Nowadays, many psychiatrists, psychologists, and counselors actually advise people to fantasize, or perhaps even have a "little fling." Advertisements and fashions all play to the lust of the flesh, making it increasingly more difficult to obey Jesus' command. Teens sometimes scorn their peers if they are still virgins by fourteen. This topsy-turvy world has completely lost its understanding of this commandment.
Much like our big cities today, ancient Corinth was filled with sexual temptations. Paul advised the brethren there to "flee sexual immorality" (I Corinthians 6:18). Too often we flee, yet leave a forwarding address! It is unwise to hang around people, places or situations that tempt us into sex sins (Proverbs 5:3-14; Genesis 39:7-12). When we flirt with temptation, we can end up as an ox going to slaughter (Proverbs 7:6-27).
Why should we flee temptations? Revelation 21:8 states that God will sentence the sexually immoral to the Lake of Fire, right along with murderers, idolaters, liars, and other unrepentant sinners. Paul adds
Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites . . . will inherit the kingdom of God. (I Corinthians 6:9-10)