THE ENGLISH MEANING OF THE WORD FORNICATION
To understand the English meaning of the word fornication we should begin with the root - "fornicate"
Fornicate "(of two people not married to each other) have sexual intercourse.
There is a huge difference between "two people who are not married" and two people who are not married to *each other*
If two people have sexual relations and one is married and one is not they fornicate - they are guilty of fornication.
If I were to have sex with someone who is not my wife we "fornicate" because we are not married to "each other" .
Because of my fornication I am guilty of adultery. The other person with whom I fornicated may or may not have committed adultery but we both fornicated.