The porneia referred to is anterior to the union in question: our Lord is saying one may leave a spouse whose unchastity is discovered to have been to a degree that throws into doubt his/her intention to commit to a permanent and exclusive union.
Apparently that is what your religion wishes it said...And what your religion twists it to say...But it doesn't say that does it...
Mat 19:9 And I say unto you, Whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery: and whoso marrieth her which is put away doth commit adultery.
It means what it says...Divorce is allowed when the wife is unfaithful during the marriage...
That makes absolutely zero sense.
Unchastity prior to the marriage can make it invalid, but sexual infidelity after the wedding is irrelevant?
The Greek work translated in KJV as fornication refers to any sort of sexual misconduct. The English word fornication implies misconduct by an unmarried person, but the Greek word is a much broader term.
Most modern versions translate this verse as: sexual immorality, infidelity, unchastity or some similar term.
Here’s how the Expanded Bible reads: “The only reason for a man to divorce his wife is if his wife has sexual relations with another man.”
Here’s a discussion of the Greek word porneia.