(Porneia) fornication is not defined as hetero-sex between a single man and woman in love. Certain Pauline comments have been interpreted as such, but it is not explicit.
Basically because passages like 1 Cor 7:
Nevertheless, to avoid porneia, let every man have his own wife, and let every woman have her own husband.
lend to that interpretation. However, a careful reading only says that without an outlet for sexual desire, porneia is a greater temptation. It does not define porneia in this passage.
Further, the author's argument is hard to follow, as they suggest that unlike other "sins" poneia involves a person's sinning against their own bodies. 1 Cor 6
12 “I have the right to do anything,” you say—but not everything is beneficial. “I have the right to do anything”—but I will not be mastered by anything. 13 You say, “Food for the stomach and the stomach for food, and God will destroy them both.” The body, however, is not meant for sexual immorality but for the Lord, and the Lord for the body. 14 By his power God raised the Lord from the dead, and he will raise us also. 15 Do you not know that your bodies are members of Christ himself? Shall I then take the members of Christ and unite them with a prostitute? Never! 16 Do you not know that he who unites himself with a prostitute is one with her in body? For it is said, “The two will become one flesh.”[b] 17 But whoever is united with the Lord is one with him in spirit.[c]18 Flee from porneia. All other sins a person commits are outside the body, but whoever does porneia, sins against their own body.
His argument as I said is hard to follow. He is at one hand saying Christians have a right to do things but there is something ritualistically, sacramentally inappropriate about porneia. He lifts the "one flesh" from Genesis and literalizes it as sexual coupling. He then uses it metaphorically as union of spirit with Christ. It adds an odd sexual element to the Christian union with Christ. It also doesn't explain how a married Christian who has sex with their partner isn't also "cheating" on Christ as it were. It's not a good argument. He also seems to specify prostitution as porneia.
As a side note, from the standpoint of the unique WT interpretation of these passages, the objection ought not apply to "Other Sheep" who do not share in the death and body of Christ.
I have resisted in this instance saying "Paul" wrote this or that because many of these sections in 1Cor show evidence of at least 2 layers of interpolation. One very early, one about a century later. That is another huge topic.
