'fornix' is not the word being used. Fornication does have to do with premarital sex. Look at Paul's teachings on marriage, or Jesus' teachings for that matter.
Fornication is not just sexual immorality or adultery, the Greek disagrees with you:
G4202
πορνεία
inappropriate contenteia
Thayer Definition:
1) illicit sexual intercourse
1a) adultery, fornication, homosexuality, lesbianism, intercourse with animals etc.
1b) sexual intercourse with close relatives; Lev. 18
1c) sexual intercourse with a divorced man or woman; Mar_10:11,Mar_10:12
2) metaphorically the worship of idols
2a) of the defilement of idolatry, as incurred by eating the sacrifices offered to idols
Part of Speech: noun feminine
Emphasis mine.
This word is used in multiple instances, like this one:
1Co 6:12 All things are lawful for me, but not all things are profitable. All things are lawful for me, but I will not be mastered by anything.
1Co 6:13 Food is for the stomach and the stomach is for food, but God will do away with both of them. Yet the body is not for immorality, but for the Lord, and the Lord is for the body.
Or this one:
1Co 7:1 Now concerning the things about which you wrote, it is good for a man not to touch a woman.
1Co 7:2 But because of immoralities, each man is to have his own wife, and each woman is to have her own husband.
Or this one:
1Th 4:1 Finally then, brethren, we request and exhort you in the Lord Jesus, that as you received from us instruction as to how you ought to walk and please God (just as you actually do walk), that you excel still more.
1Th 4:2 For you know what commandments we gave you by the authority of the Lord Jesus.
1Th 4:3 For this is the will of God, your sanctification; that is, that you abstain from sexual immorality;
1Th 4:4 that each of you know how to possess his own vessel in sanctification and honor,
1Th 4:5 not in lustful passion, like the Gentiles who do not know God;
1Th 4:6 and that no man transgress and defraud his brother in the matter because the Lord is the avenger in all these things, just as we also told you before and solemnly warned you.
1Th 4:7 For God has not called us for the purpose of impurity, but in sanctification.
All those seem pretty clear to me. Lust is condemned too, after all.