Sex without marriage is sex without commitment. The act of sex usually makes people want security and stability - this is particularly true of girls. If this is not obtained, they can become depressed, and even suicide attempts may follow. Guilt after casual sex can damage people, and any future marriage. Cancer has been shown to follow promiscuity too.
Sex can follow, beautifully and easily, if the friendship and caring are there first.
God has a better plan As God invented sex, He understands it power, and the temptations to use it wrongly. Virginity in the girl and the man is the best place to start a marriage. It proves a person can be faithful and it is the best wedding present of all. If virginity has been lost, the next best thing is being sorry, and having no more sex till marriage. Engagement is not the same as marriage - it is easily broken.