The positive effect of a soul tie: In a godly marriage, God links the two together and the Bible tells us that they become one flesh. As a result of them becoming one flesh, it binds them together and they will cleave onto one another in a unique way. The purpose of this cleaving is to build a very healthy, strong and close relationship between a man and a woman.
Matthew 19:5, "And said, For this cause shall a man leave father and mother, and shall cleave to his wife: and they twain shall be one flesh."
The negative effect of a soul tie: Soul ties can also be used for the devil's advantage. Soul ties formed from sex outside of marriage causes a person to become defiled:
Genesis 34:2-3, "And when Shechem the son of Hamor the Hivite, prince of the country, saw her, he took her, and lay with her, and defiled her. And his soul cleaved unto Dinahthe daughter of Jacob, and he loved the damsel, and spoke kindly unto the damsel." This is why it is so common for a person to still have 'feelings' towards an ex-lover that they have no right to be attracted to in that way. Even 20 years down the road, a person may still think of their first lover... even if he or she is across the country and has their own family, all because of a soul tie!