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?
I take it that you’re not a practicing Catholic since you had a divorced women as a girlfriend.
Is that correct?
To state that porneia is meant to be a just reason for dissolving a valid marriage would empty our Lord's response of its meaning, merely placing him with the school of Shammai. Note also that his disciples understood this as the meaning of our Lord with their response, "If that is the case of a man with his wife, it is better not to marry."
Additionally, as Catholics we also look to Sacred Tradition as well as the Bible for guidance. This has always been the Catholic understanding of our Lord's words and as such is as much as an infallible guide as the Bible itself.
I am sorry about the history that you have presented about your parents and your former girl friend. But the question is always are we to be faithful the words of Jesus Christ. Following the Gospel is not always easy but if we open our hearts to the grace of God and seek always to do his will, and not our own, he will give us whatever graces we need to carry our crosses.