I would contend that any successful attempt to find a "right to marriage" in any form under the U.S. Constitution is going to have very interesting consequences. If such a right exists, and states are forbidden to legislate against "gay marriages" on this basis, then states would also be forbidden from legislating against any other kind of "marriages," including the following:
1. A marriage involving more than two spouses (Utah can go back to the days before they were admitted into the Union),
2. A marriage involving close family members (no more incest jokes about Appalachia),
3. A marriage involving a person and an animal (if you think the future of Social Security was in bad shape now, wait until pets start collecting survivor benefits),
4. A marriage involving an elderly retiree and a 5 year-old family member (to allow the youngster to collect insurance, Social security, and pension benefits for decades after the other "spouse" dies),
4. A marriage involving large numbers of committed Christians who use such a travesty in a cynical, manipulative manner to their own advantage (I'll probably never pay another dime in income taxes for the rest of my life, since I could easily come up with 30 or more "spouses" to put on my income tax returns).
Providing insurance benefits to same-sex couples is one thing, but I suspect that any attempt to change the legal definition of a marriage will stop in its tracks once Items #4 and #5 become a common practice.