A psychologist I know struck up an affair and eventually married one of his patients, a young woman his granddaughters age. I asked him why he would violate his ethics to do such a thing. His answer: "What could I do, she is so pretty and wanted to be with me."
In Isaiah 52:11 we read: Depart! Depart! Go out from there, Touch no unclean thing; Go out from the midst of her, Be clean, You who bear the vessels of the Lord. (NKJV) This command was to the exiles in Babylon to come out, and to leave Babylon behind. Paul warned the church not to be unequally yoked with unbelievers. Peter said: "Beloved, I beg you as sojourners and pilgrims, abstain from fleshly lusts which war against the soul," 1 Peter 2:11 NKJV
The time for Israel to leave Babylon came but most of them stayed in Babylon. They were so entangled in commerce and culture that they had no desire to leave. Depart! Depart! was the command to come out, not just to leave Babylon but also not to take Babylon with them.
We also need to be in the world without being of the world. But, it is so easy to entangle ourselves in commerce and culture and find that we can't say no to the unthinkable. If we bind ourselves to unbelief we will soon be led away captive to evil. Come out from the World, touch not unclean thing.