I share Mark Silk's reluctance to engage in public disagreement but I have an additional reason that Silk does not. He cites our friendship, which, I also treasure. But, I also know that Silk is far more learned than me so disagreeing with him is perilous as well as distateful.
That said, his defense of Gary Wills entails accepting the distinction Wills makes between the Church's teaching on the sacramentality of marriage and the medieval development of specific rites of the Church that surround that sacrament. The fact that there was no rite before the late Middle Ages does not mean matrimony was not a sacrament. As Aquinas explained, it is the consent of the two individuals, and the consquent consummation of the marriage, that are essential for its validity. At the sixteenth century Council of Trent, the Council fathers determined that a marriage between Catholics must be within the canonical and liturgical norms of the Church to be valid. Trent made no such judgment about non-Catholic marriages which are presumed to be valid. I confess I do not understand the reasoning of the Council fathers at Trent in this regard. But, back to Wills - his argument is wrong in suggesting that the Church one fine day decided to make marriage a sacrament. It is more accurate to say that the Church, over time, came to recognize the sacramentality that was already there.
Silk is correct that history matters. Again, and contra Wills, the fact that the Church's doctrine developed in this regard is no cause to reject it. Indeed, the Church lives out its mission in history. Many things went into the Church's evolving attitudes towards marriage, not least St. Paul's clearly expressed marital analogy for the relationship between Christ and his Church. The fact that the doctrine developed is obvious. But, I think that supports my view not Wills'.