That's because 'fornication' is not the mere act of sex, but that of sexing prostitutes in temples for the worship of local deities.
Sex was and has always been rampant in the world and does not keep one out of heaven, as it is a basic part of our human design. Nothing we do sexually, as was once pointed out by a mentor, is unnatural. "If it were unnatural," said he, "then we couldn't do it." Made sense.
However, as sex is often the vehicle we use to ruin relationships and gain control over others, it is as such outlawed by puritans afraid of their own burning desires.
So temples to alleviate and divert those desires to the worship of minor gods, were ubiquitous in ancient lands, with each temple identified by what we now call a Totem Pole, a hewn oak tree—an Elah—carved with images of local gods.
A traveler from one town to another who would see such a post—Ashertroth Post—outside a temple, was obliged to pay a visit within, where the happy traveler would relieve himself with boys and girls for proxy sex to the deities of the area, thus enslaving that person to the aforementioned fallen angel spirits.
Standard practice for the day, it was for that reason the early gentile church was warned to not 'fornicate' lest it be trafficking with God's blasphemers.
The observer, long a student of demonology, once quizzed a young lady who'd gone into an unplanned hypnotic trance -- her right eye completely dilated and her left focused to a pinpoint as if betraying brain-damage -- her familiar spirit took over, and it was to that spirit he talked.
Answering my questioning in this state, she admitted that her familial spirit had been a prostitute in an ancient temple of fornication, under authority of a senior evil spirit whose name we'd all recognize. As her parent's house proudly displayed a collection of pre-Colombian artifacts—household-gods—I was not surprised.
However, her young husband found her sexually inept and argued that such should have experienced her -- yet, what woman hasn't just lain still and taken it when raped by a lout? I found her responses quite believable.
Then some years later, a nipple-ringed, tatoo'd goth girl who kept Ouija boards as kitchen table placemats, proudly expressed having sexual trysts with "Jesus Christ." (or rather his incubus).
Expecting me to register proper Christian horror at an abomination, her face fell as I nonplussedly explained her spiritual fornication with a demon masquerader. And as I've seen in other instances, the demonically-inclined are excellent reactionaries of truthfulness, so her crestfalleness proved me out.”The observer knew she'd been fooled because he's not only best friends with the real Son-of-God, but is presciently aware of sex-demons from books, films and real encounter -- and from discerning nunneries as Christian harems often visited in the night by incubi. For purposes of fornication.
Now none of this is what we were taught to believe, now is it?