The argument I never hear, is that gay marriage might work. By 'might work' I mean that gay marriage might have the stability, 50%, of straight marriages.
To me the most powerful benefit of marriage is the pairing and general caring of older married pairs.
Care for the elderly is a vital and costly element in our society. Anything that provides care for the elderly should be given serious consideration.
Additionally, I never hear any discussion of the need for prenups, pre-nuptial agreements, in connection with civil unions. Why? A civil union in California is identical to a marriage. In both cases, the absence of a prenup, means all property acquired during the period of the civil union is joint property.
In more than a few cases I know of gays in a relationship where one has a great deal of money and income and the other has little or none. So, the absence of discussion about prenups strikes me as a serious social failing.