A major term in scripture that is translated "sexual immorality" is porneia. What the purest definition means is the selling of your sexuality, and was known as describing going to the prostitutes at the temple of an idol. Thus it wasn't so much the sex, but it's link to idolatry in this regard.
"Fornication" is a Latin word, and didn't enter biblical thought until the Latin Vulgate. Brought on by the Roman Catholics trying to gain more control over people's lives. The best way to do that? Make people believe that they would go to hell for doing something.
The other thing is to look at all the men who had multiple wives, and slept with women who were not their wives, and see that God did not "smite" them. Good smote King David one time, because of his adultery with Bathsheba.
Adultery is akin to idolatry. Which is the replacement, disrespect, and betrayal of the spouse. Cheating is about deceit and unfaithfulness. Where as hotwifing is about openness and devotion to one another.
Another simple thing to do is ask this question. "If one man and one woman was the only way to go about this, why didnt He just say that?" I mean, instead there are guidelines, and standards to uphold that would insinuate that something like this lifestyle is legit.
My personal opinion is if Hotwifing and Swinging brings a husband and wife together, makes their marriage stronger, creates greater depth of intimacy and communication, brings joy and sexual fullfulment with each other, then this is positive.
I believe that we are sexual beings and that God gave us each other to be pleasing to God, worship God, and be pleasing to each other.
Swingers that I have met, in general, are some of the most giving, friendly, caring, and genuine people I’ve met. They enjoy pleasing others and their mates.
There are exceptions to everything, of course, but the saying “Couples that play together stay together” appears to be true, in my opinion.
We are alive such a short time.
If swinging and hotwifing activities bring joy and pleasure to “all” involved then this is positive.
If swinging and hotwifing is done under intoxication, duress, intimidation, decet, and anyone involved feels bad or hurt for participating it pulls a marriage apart then its very bad, the definition of sinful in my opinion