Some that I knew in the past would advise that you separate, marry (civil marriage), and then come back together.
However, Paul addressed unmarried couples who had trouble controlling themselves. Paul's advice wasn't that they separate. Paul wrote...
1 Corinthians 7:8-9
English Standard Version (ESV)
8 To the unmarried and the widows I say that it is good for them to remain single as I am. 9 But if they cannot exercise self-control, they should marry. For it is better to marry than to burn with passion.
An unmarried couple that are having trouble controlling their desire for one another shouldn't be separated. They should be scheduled to marry. Paul has some real practical advice. So... get "married" (private or civil).