Marriage is the deepest and most intimate of all human relationship. It is written, "Therefore shall a man leave his father and mother, and shall cleave unto his wife and they shall be one flesh. And they were both naked, the man and his wife, and they were both ashamed" -Gen 2;24-25... That's intimacy.
This kind is only permitted inside of the marriage institution. outside of the marriage, it can be uncleanliness, fornication, adultery and such like. in this definition you will observe that their spirits become one, their souls become one and their bodies become one, they cleave to each other totally. "... and a threefold cord is not quickly broken" -Eccl 4:12. This is fusion.