I just had a college student e-mail me this question: "I've always been told that sex before marriage is wrong, but I can't find in the Bible where it explicitly says it's wrong. Why as Christians do we say it is? I don't doubt that it probably is a sin, but i don't know why."
Here is my response:
We know sex outside of marriage is wrong for a few reasons and probably more I don't have here:
1) In the Genesis account, God established sex within the confines of marriage (Gen 2:24; "one flesh").
2) Read 1 Cor 6:13-20 carefully and notice Paul's argument about having sex with a prostitue (or a woman outside of marriage). He says you become "one flesh" with her. Then he appeals to Genesis 2:24 and how God established sex for the marriage relationship.
3) The biblical terms "sexual immorality" or "fornication" include all manner of sexual activity outside of marriage between a man and a woman. That is how people at that time understood these terms. This would include adultery, homosexuality, or sex before marriage (see 1 Thess 4:3-8; 1 Cor. 6:9, 18-20, ).
4) In 1 Corinthians 7:1-2, Paul says that marriage is the proper place for sexual expression, and that because of the temptation to sexual immorality (sex outside of marriage), each person should get married (in order to biblically express their sexual desires).
5) Why would Jesus say it is a sin to look on a woman and lust after her in your heart if it was permissible to have sex with her? (Matt 5:28)
Another practical argument that I have made is that sex outside of marriage is like asking a girl for the benefits of marriage without the commitment. What happens if you break up? How many girls will you have sex with? God thinks sex is a big deal, and he reserved it for the relationship of deepest commitment and companionship.