Ross Douthat on his New York Times blog highlights Eve Tushnet's response:

I thought these points from Eve Tushnet were worth highlighting:

1) … I was struck by the conflation of forgiving adultery and understanding nonmonogamy such that there is nothing to forgive in the first place. These actually seem to me like opposite moral positions, but both Savage and Oppenheimer (in his role as sympathetic conveyer of someone else’s position) consistently conflate them.

2) … On a related note, I’m struck by how the only players in this story are a) the adults (in the magazine story as printed it’s really only the adults in the marriage, but even in Oppenheimer’s comments here he only looks at e.g. the mistress, her boyfriend, or other adult parties who might be affected emotionally) and b) the children within the marriage. Have we really forgotten that sex still makes babies? There will be children of affairs, too, and so framing (heterosexual) adultery as a stay-together-for-the-kids plan strikes me as a great way to enhance the inherent inequality between children of the marriage and those outside it. Out-of-wedlock children, in this worldview, become unfortunate side effects of the sexual license designed to protect the marital children.

3) … But couldn’t we all just be rational actors, contracepting demi-perfectly and backing it up with abortion? Then no worries!

But of course the whole weird premise of Savage’s claim is that eros is so powerful and irrational, sexual fulfillment such an obvious non-negotiable, that… we should talk things out like rational adults before we get married and then stick to our rational rules and goals. Eros is simultaneously overwhelming—breaking down the strong norm of marital fidelity—and easily-tamed, contained within little well-contracepted well-communicated honest and generous mini-affairs.

Eve in turn points to this great piece on Dan Savage's ethics.