That's what I always believed about that verse, also infidelity sounds the same as immoral to me, is it different?
“Do not be deceived: Neither πόρνοι (
pornoi ) fornicators nor idolaters nor μοιχοὶ (
moichoi ) adulterers nor men who have sex with men. “ 1 Corinthians 6:9
Note different words are used to delineate the two.
It’s the difference between legitimate union, marriage and illegitimate union, fornication.
“I tell you that whoever divorces his wife, except for sexual immorality ( except in the case of a fornication relationship ), and marries another woman commits adultery." Matt 19:9
It’s not saying if your wife commits adultery you can divorce her and remarry.
As someone astutely pointed out, if a wife was caught in adultery, she would have been stoned to death rather summarily under the law of Moses, and the man would have been free to remarry anyway.