Summary:
God has such a high value on sex that He designed it to be just for marriage.
The argument starts in Genesis. God makes Adam and Eve, marries them, and they enjoy sex as an expression of their unity: 'For this reason a man shall leave his father and mother and be united to his wife, and they will become one flesh.' Gen 2:24 The implication is that you can't become one flesh physically (ie in sexual intercourse) without first being joined before God in marriage. This makes sense in that sex is a picture of giving yourselves completely to one another- this just isn't appropriate outside the safety net of marriage, where you have made a lifelong commitment to each other.
The high value God places on sex within marriage is emphasised throughout the Old Testament- most notably in the command against adultery (Ex 20:14). If you engage in pre-marital sex, you are in one sense being unfaithful to your future spouse. If the person ends up being someone you marry, you have still spoilt something special and unique meant for marriage only.
There are people in the Old Testament who disregard God's design for sex and marriage, and the result is always spiritual disaster. Look at Solomon: he had a ridiculous number of concubines (300!), and they had a terrible influence on him. He also married 700 women, and they 'turned his heart after other gods, and his heart was not fully devoted to the Lord his God' (1 Kings 11:4). Solomon had built the temple and was gifted with wisdom from God, and yet in this matter he was blind to the warnings he was given, and so his kingdom was torn away from him (1 Kings 11:11). This ties into the idea that Christians going out with/marrying non Christians is spiritually damaging, because you are essentially uniting yourself with someone who is spiritually dead (see Ephesians 2 for the strong contrast between Christians and non-Christians, and 2 Cor 6 for instruction on not being 'yoked' with unbelievers).
The New Testament letters speak a lot about sexual purity. There are several key passages where 'fornication' (ie sex before marriage) is condemned (1 Cor 6:12- 7:40; Eph 5:1-7; 1 Thess 4:1-8), and in Hebrews 13:4 it says that the marriage bed should be kept pure, for God will judge the sexually immoral. The key emphasis in all these passages is that we've been saved by God's grace and set free from the crippling, disabling power of sin over our lives. Therefore our whole lives as Christians are about using that power to resist sin and live 100% for Jesus instead of being ruled by passion just like non-believers are.
When people have problems with what the Bible teaches on sex, it actually points to a bigger problem: their attitude towards God. Too often we are driven by our own desires for a relationship or physical intimacy, instead of putting God's agenda at the top of our priorities. If you're a Christian and you know another Christian who is struggling in this area, encourage them to draw closer to God. If you can, suggest meeting up together to read the Bible. Working through a short letter like 1 Thessalonians would only take a few sessions, and all you'd have to do is read it and discuss it. If they're really serious about being a Christian, they have to accept that it intrinsically means giving up what you want, and going God's way instead. And the joy of being a Christian is that God's way is so much better!!! As Paul writes, those things (sexual immorality included) lead to death. Why would we want to go back to them? (Romans 6:21)
Ultimately, we need to realise that God loves us far more than we comprehend. He loved us enough to send Jesus to die for us. Won't He give us anything that's good for us? Would He with hold the best from us? Of course not. (see Romans 8).
Therefore the Bible's teaching on sex and marriage is there to help us. God is FOR sex (he invented it!!!) and He has such a high value on it that he reserves it for those who are married. People who sleep with others before marriage are de-valuing sex. God's way is best because it preserves society and the family unit- look at the result of people sleeping together outside marriage: STDs, AIDS, broken homes, kids without fathers... And I hate to say it, but usually the girl in the relationship is the one with everything to lose and nothing to gain. A guy can sleep with someone and walk away; the girl ends up way more emotionally scarred and potentially with a baby to look after.
I think if a guy really loves you, he'd be willing to wait : Christian OR non Christian. But the real sticking point about Christians going out with non Christians is that they can't be a gospel team, working together to serve Jesus. One serves Jesus; the other belongs basically to the devil and is spiritually dead. I think one of the greatest blessings of Christian marriage is that you keep each other going with Jesus, pick each other up when you're down, lead each other back to the cross. However attracted you are to each other, it's the spiritual bond you share that will keep you going in the tough times.
