Paul’s instruction to separate did not apply to the fornicators of this world – people at work, in your place of employment, people in your college, family members who are unsaved. You cannot have nothing to do with the fornicators of this world, or the covetousness, or extortioners, or idolaters; for obviously you would need to leave the world.
At times in history, Christians have made the mistake of embracing monasticism. That is indeed an attempt to go out of the world, motivated by the very same mistaken attitude that Paul is correcting here. No, the church is left in the world to win it for Christ. Every member of the church has come out of the world, and the wonderful work of salvation does not stop with us. We in turn are to reach out to others who are as we once were.
You have to be civil to all people and courteous and seek to reach them, and sit in the same office, and the same family. Of course: sin is all around us, but in the church has to be purging.