On the question of sex before marriage, it is really best answered by understanding the nature and purpose of sex. God gave humans the wonderful gift of sex to be used in marriage to bring intimate union and fellowship between a man and woman and for the bringing into the world of children. This is clear from Genesis 2:20-25 (esp v.24). Jesus reiterates this in Matthew 19. Of course, this does not restrict people using sex in this context, but that is the way God designed it to be used. That is its nature and purpose. If I can illustrate: you can use a knife to open a paint tin, but that is not its purpose and you will end up damaging the knife and perhaps the paint tin. You can have sex with someone you are not married with that is not the way God intended that great gift to be used and even non-Christian research shows that people who live together and have sex together before marriage divorce more readily and have less satisfying marriages. I would argue that like greed, anger, lying and stealing, sex before marriage is sin.