Mary Norton and Wendy Becker — both from the famously underprivileged professional class of college professors — have made substantial progress in their quest to disprove all of those same-sex marriage advocates who swore that judges would not be able to export the marriage policy that they (the judges) had created in Massachusetts. In a letter to the Providence Journal in which the couple responds to a letter to the same publication by Providence Roman Catholic Bishop Thomas Tobin, they give some explanation as to their motivation:

We have been involved in a loving relationship for 18 years. We are raising two wonderful children who are growing up to be compassionate, inquisitive, and kind. We wanted to be married to provide our children with the legal protections they may need and provide our relationship with the security it deserves.

Being sufficiently charitable to see this paragraph as more heartfelt than a regurgitation of movement talking points, I find its thematic transition somewhat perplexing. The first sentence of this truncated quotation is presented as if to indicate that the women’s relationship has been as committed as marriage for almost two decades, yet the last sentence insists that they require marriage in order to make their relationship secure.
I understand that proponents of pushing same-sex marriage through the judiciary find themselves having to maintain a careful linguistic balance. On one side, they must make an essentially moral argument in order to leverage the strength of civil rights sentiment. On the other, however, they must couch their goals in the language of cold law and civic interests; otherwise, it would be more difficult to hide the reality (and it is a reality) that they ought to be working through the legislative process. In their own effort to strike this balance, Norton and Becker only point the way to the public interest that both makes marriage crucial to public well-being and allows it to remain exclusive to oppose-sex couples without amounting to invidious discrimination.
If marriage is intended to encourage stability, then it is implicitly geared toward relationships that:

1. would cause harm were they to end and
2. are in danger of instability.

On the first point, the only circumstances that fall within the government’s scope to care is when the end of the relationship would affect children. Yes, the public has an interest in encouraging mutual care, but not only is such a goal arguably beyond the boundaries of vagueness past which the government is simply meddling, but it also offers no justification for excluding relatives or those who wish to form groups of larger than two people.
On the second point, the targeted couples are plainly not those whose commitment is ensured even without the encouragement of the marital institution. And the only relationships into which children are likely to enter (bringing with them the overriding public interest) without a previously formed commitment are those in which the partners are capable of creating children through their own actions. A long-term couple that has gone through the process of adopting children is not likely to be under the same threat of insecurity.
The argument is well worth considering that non-procreative opposite-sex couples are allowed to marry and present no relevant distinctions from same-sex couples. (In this area, I agree with Bishop Tobin on matters of morality, but we apparently differ on how morality ought to affect and operate with the law and the legislative process.) The observation that ultimately undermines that argument, however, is that same-sex couples would inherently sever the link between marriage and procreation, while non-procreative opposite-sex couples do not.
Norton and Becker end their letter with a question that presumes too much: “What could the Church find immoral in protecting children and creating secure families?” Children are most protected by a culture with the confidence to insist that their creation — not a set of legal rights and privileges — ought to be inextricable from their families’ security.