Sleeping together before marriage is called funication, most people has gone too far in this act without knowing what they are getting themselves into, they think it's pleasure or it's the right thing to do to show love or to prove they love someone and will want to marry the person.
But the bible really condemns it, it's an abomination in God's sight, marriage is a beautiful thing to engage in that's if proper things are done and if proper steps are been taking.
Nor matter how engaged you are with the person sin is sin, until you are married then it will be ordained by God.