I believe you are mistaken unless you have an authoritative source.
I have never discovered one in my years of searching.
Nor is that the Catholic definition of “fornication”.
You have defined “a human act of fornication” not “fornication” per se.
The “matter” of a human act is rarely defined by the interior apprehensions or deliberateness of the participant. That is taken care of by the concept of culpability - not by redefining the matter itself.
Hence such couples are technically fornicating (defined as sex between those who are not married before God). However they were in good faith so no culpability attaches at all.
Is it adultery if my wife’s twin sister tricks me when she stays over for the summer?
Yes it is, technically. However it is fully non imputable because I was completely unawares.
You will not find a Catholic definition of “fornication” that includes full knowledge and consent as part of the very definition. That is added on afterwards when we look at whether it was a complete “human/moral act of fornication”. Fornication is but the object or “matter” (a grave one at that) of the human/moral act. I believe you have confused the two.
Non imputable, by definition, means awareness or consent is defective to some degree and the act is not a full human/moral act. It may not have been a “human act” act all if I was blind drunk or totally ignorant of its true nature. Where there is no human act there can be no personal sin.
Hence non imputability, non culpability.
Was Oedipus in an incestuous relationship when he unbeknowingly married his mother?
Yes he was. But it was fully non imputable.
The “matter” of a human act is its object which not surprisingly is “objective” and cannot be re-defined simply by the views or deliberateness of the agents involved surely?