Reason #1 - God Tells Us Not to Have Sex Outside of Marriage For sexual immorality is a sin against your own body. Here's food for thought: If your partner wants sex before marriage, consider this a warning sign of his or. The Bible uses the same word talking about reasons for leaving a marriage, which sex with a woman besides your wife is clearly not premarital. The Bible does not call sex before marriage a sin against God. The relationship between sin and sex before marriage is a new concept of Christianity. The only.