So what does Jesus mean when He says, "except in the cases of porneia"? A better understanding of the Greek is that it means "unlawful sexual union" --- i.e., that the supposed first marriage itself was porneia, unlawful.
That makes sense because (1) it's a more accurate translation, and (2) in the practical sense, Jesus could not have said that anyone who wants to get out of a marriage, just has to commit adultery, and then, voila, divorce is possible. This would positively reward people for adultery, by dissolving their marriage, which is what they wanted in the first place.
Besides, if adultery could dissolve a marriage, it would contradict what Jesus said about marriage being a bond which "no man can put asunder." If you can put it asunder just by going ahead and committing adultery, then every marriage is dissolvable, and rather easily at that.
Both:
Jesus words included all sexual sin that violates the sacred bond of marriage - Clinton wouldn’t like it. That said, actual intercourse outside of one’s marriage is included in the term, as well as many other acts.
“i.e., that the supposed first marriage itself was porneia, unlawful.” - eisogesis that isn’t in the text or context.
Christ doesn’t mention annulment as a result, but divorce, reiterating the instructions God gives in the Hebrew Scriptures. There is no annulment in the Scriptures. God hates divorce, but allows this instance “for the hardness of your hearts”.
Practical terms: divorce is allowed when sexual sin outside your marriage has broken the bonds of one flesh.
Paul elaborates in I Cor 7 to deal with your other concerns.