Matthew 19:9

[New Jack Swinger]

“And I say to you, that whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery: and he that shall marry her that is put away, committeth adultery.”

What did Our Lord mean when he said “except it be for fornication”?

[DavidObeid]

It is a reference to illegitimate or invalid unions.

He does not mean that a divorce can be obtained if there has been some post nuptial infidelity.

[Matthew]

There are quite a few explanations of this verse. Dave has given one explanation which I think I prefer.

Another is that he is referring to the situation where a one party commits adultery and they can live apart but without contracting a new union which situation St Paul explicitly refers to.

[DavidObeid]

That's right. And either way, after a valid marriage, there can never be a re-marriage to someone else.