Having sex before marriage is not a sin. In ancient times, people got married young, had sex, the men went to work, and the women stayed at home raising their children. It was all out of convenience.
Marriage on the other hand, is seen as Sacred. After people decide to get married in the church, their commitment to each other is blessed by god through a priest. The church preaches to both partners to honor that marriage and to make it last until death do them part.
Most conservatives, christians and none christians, insist that one should remain virgin until marriage.