I did some research on the term back in the day (1980s) and decided it was somewhat of a misnomer.
There certainly is an on-going non-physical linkage between 2 people who have sex (physical linkage). Paul warns us of this:
1 Corinthians 6:16 Or do you not know that the one who joins himself to a prostitute is one body
with her? For He says, “The two shall become one flesh.”
Clearly thru scripture the husband and wife "oneness" continues on beyond just the physical act of sex, here Paul said the same thing happens with the most non-commital sex there is: prostitution. We of Trinitarian belief understand that there is one God but there are Father Son and Spirit. We often look at man made in that image as Body Soul and Spirit. But just as OT Judaism did not see God divided that way, they also did not see man divided that way either. "Body" or "soul" often indicated the WHOLE PERSON.
That being so, then the physical coupling in sex produces an ongoing linkage in
both the spirit and soul realms. (not JUST the soul)
Beyond the statements of "one flesh" that can be understood to include both soul and spirit, the bible has little else to say on this subject.
But when you look at the prohibitions against non marital sex from the perspective of this ongoing linkage, it makes sense. God wants us to have linkage; but only according to how He designed us. We were
not meant to have that kind of links to people we are not married to, or with people of the same gender or with animals. (or in the case of the guys who visited Lot - with angels)
IOW, it may not be the sex
per se, but the linkage that results from that sex that is the real problem.