The church's role in marriage is essentially decorative. That is what solemnization means — to "dress up" with "solemn" ceremony. The church does not "make" the marriage; that is the work of the couple, whose vowed consent is the crucial element in the matrimonial confection, much as some hold the words of institution to be in the Eucharist. But the church's witness and blessing do not constitute the marriage. Rather they add a level of decorum and ceremonial gravitas. It is a matter of style rather than content, and the Episcopal Church (at least) holds that a couple is just as married in the city hall as in the cathedral.
In all of this the church follows the lead of its Lord, who didn't, as far as we know, ever do more in this regard than "adorn" a wedding feast at Cana of Galilee.
This is what makes the decision to allow clergy to bless same-sex unions or marriages, but not to officiate at the exchange of vows so very odd. The officiant over the vow exchange literally adds nothing to the vows other than the decorous and solemn ceremonial; the blessing, we hope, does add some real substance to the mix, and is surely the properly ecclesiastical share in the event. To allow clergy, in these cases, the exercise of the churchly ministry and bar them from the essentially civil functions of prompter and recorder appears to give more weight to the latter role than it warrants. One might well say that the blessing is as much of a "solemnization" as the assistance given in the exchange of vows and the pronunciation that what everyone has just witnessed has in fact taken place.
Tobias Stanislas Haller BSG