Not sure what you mean by “exclusion clause.”
Nothing invalidates a valid marriage except death.
Moreover, what Matthew 19:9 says is not merely “unchastity” but “fornication” specifically, i.e., sexual intercourse outside the marital act (i.e., adultery). And it is generally understood that adultery dissolves the marriage quoad thorum but not quoad vinculum, that is, with respect to the bed but not the bond, that is, what adultery does is absolve the aggrieved spouse of their obligation to render the marital debt without otherwise freeing them to offer themselves in marriage to another.