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.
I understand that the churches who teach this interpretation, hold that every marriage is potentially divorceable. In other words,there's no such thing as a truly binding marriage in the eyes of God. Do you agree with this?
And do you think that an act of sexual infidelity confers the right to divorce one's spouse? (Cheers and high-fives from adulterers all around!) This is the necessary corollary of your "practical terms" which I quoted verbatim, above.
I can't see how this could be Jesus' intent. Christian marriage implies the restoration, by Christ Himself, of marriage to its original indissolubility, so that there can never be a complete, absolute divorce (with the right of re-marriage) after a valid marriage has been consummated, as long as the spouse is still living.
Adultery isn't in the text or context. Read the Greek; it's not there.