And would adultery on behalf of one of the parties invalidate that marriage, especially if they continue to unrepentantly do so?
See post 8, point #1. In reading the Greek text, I simply can’t see the word porneia meaning such a strict definition of invalid marriages, especially when it’s used in so many other ways in the other 25 places it’s used in the NT.
This seems, to me, like deciding the answer before examining the evidence.
I think you’d have to go to Jesus Christ for an answer to that. And the core if His teaching is, “What GOD has joined together let no man put asunder.”
So the only question would be, “Did God join this husband and wife together?” In other words, was it a valid marriage at the beginning, when the vows were exchanged? If so, it is God Himself who has made the marriage bond indissoluble -— no matter what goes on subsequently.
Even if the couple has to physically separate for grave reasons -— say there is alcoholism, addiction, adultery, abuse, something that makes living together untenable -— this cannot dissolve a bond made by God. The innocent spouse is called to remain maritally faithful even when living apart.