As I stated before, God designed sex exclusively for marriage. However, in our culture, the majority of us (Believers included) are having sex outside of marriage. What are the dangers of continuing this practice? The answer is "soul ties".
"Do you not know that your bodies are members of Christ himself? Shall I then take the members of Christ and unite them with a prostitute? Never! Do you not know that he who unites himself with a prostitute is one with her in body? For it is said, "The two will become one flesh." 1 Corinthians 6: 15-16
In this scripture, Paul is warning men (it goes for women as well) about the dangers have having sexual relations outside of marriage. Here, God's people are warned against sleeping with prostitutes/harlots and the ramifications of doing so. This woman may not be a "prostitute" in terms of standing on a corner and selling herself, but it describes any woman outside of God's ordained marital relationship.
Paul elaborates on this situation by describing how sex unites you with another person in body "the two shall become one flesh". The term "one flesh" goes beyond penile-vaginal penetration but it describes a connection at the very soul of two individuals. The Hebrew word used to describe flesh is "basar". Basar describes a feeling of of kinship/blood relations. It comes from the word that means "to bring tidings". This "kinship" was exclusively meant for uniting a married couple. It is not the marriage ceremony that make one "part of the family", it is the sexual act of joining the man and woman into one flesh.
Kinship describes having a similar character and origin. People who are kindred have a common tie. They have the same nature and are united in harmony. What ever happens to one person effects the other. However, if you are having a feeling of kinship with someone who is not meant by God, this is dangerous territory. The illicit soul tie will create a false connection. The bond of kinship, harmony, and love may seem to be there however it is based on falsehood. This falsehood is merely a fleeting feeling of infatuation. This is how people end up marrying the wrong person, or staying in ungodly relationships.