The family is an institution that God himself instituted way back in Genesis, at the Garden of Eden. Adam and Eve became the first husband and wife and they were joined by God himself. It is for this reason that marriage is a sacred union that should be done by a man and woman who loves each other.
Although we live in a time where marriage is not given too much importance, and sexual intercourse is seen as a casual thing, it does not change the fact that marriage is something that God requires before couples take part in such activities.
If we look at the pages of the Bible, sexual intercourse apart from marriage is considered fornication or sexual immorality. And that is just the beginning of it. If you want to know more what Scriptures say about having premarital sex, then here are some verses.