Your reference is to porneia, which is not sexual immorality in the first marriage but if the first marriage itself was illicit and thus invalid.
But like all sins, even adultery is not beyond forgiveness. I would personally require confession and absolution, but I will not deny a repentant sinner the grace of God.
It also requires a firm purpose of amendment, i.e. to stop sinning. Those who remain in an adulterous, and thus invalid, second marriage with the intent to continue in adulterous sexual acts lack this necessary requirement.
1: Do you have proof of your claims? From what I’ve learned, porneia is a catch-all for sexual immorality in the Greek language. It’s used in that sense in other places in the NT as well. A quick hunt of Greek lexicons (Thayer’s, Strong’s), as well as another quick glance through its 26 appearances in the New Testament, and I can’t find a single place where it means definitely invalid marriages and most definitely NOT sexual immorality.
What evidence are you citing?
2: If the first marriage is invalid, then why does Jesus talk about it ending in divorce?
3: A second marriage may be entered into sinfully but that doesn’t make it not marriage. David married Bathsheba despite the horrendous sin that had been committed in order to get into that marriage. But the marriage remained, despite that.
4: At the risk of sounding like a liberal heart-string puller, let me cite for you two IRL examples.
My last girlfriend, before we broke up, had been divorced. Her husband had cheated on her multiple times (with multiple sexes) and was even in prison because of being a peeping tom. He broke the marriage vows first; why should she still be beholden to them? (And before you answer: Jesus said so, I still want to see proof that the word means what you claim it means)
Additionally, my mother slept around on my father for many years, and he tried very hard to keep the marriage together until she finally served him divorce papers and threw him out of the house, and then she remarried the guy she was having the affair with almost right after.
Is my father still bound to his original marriage vows even after what happened, and why?