Presumably the sin could also be absolved by abandoning the second wife. Although I personally find it difficult to see how two divorces are more moral than one.
I thought it was interesting how the author beat around the bush as to what the answer to the problem he addresses really is. Reading between the lines, he is advocating the abandonment of all Church teaching as to the sinfulness of sex outside of marriage. I'm not sure why he didn't just say so.
Where did you get that idea? He said, as I read him, that the Church has to be willing to engage in a discussion of WHY every sexual act not oriented to the conception of a child is mortally sinful and not simply appeal to authority. External authority is the weakest rationale behind any action; it may work, for a while, but people have to be able to reconcile morality with their own experience or they'll never ultimately adopt the morality.
"Do this because I say so" may have worked and may still work with uneducated peasants, but people who can think on their own will simply walk away from that kind of argument (which really isn't an argument at all).