Constitutionally, I can see no valid argument for denying gay couples equal rights under the law. In the absence of an amendment to the constitution defining marriage as between a man and a woman, it seems obvious denying the rights of gays to "marry" is obvious.

The Constitution is silent on this issue- so there is no right for it or against it. Gay marriage contributes very little to society at large and it seems that there may be some long term social damage down the road. Like single parents and divorce often are excellent predictors to a child's failure in life (teen pregnancy, drug addition, alcoholism, crime).

Other than offically legitimizing homosexuality gay marriage doesn't do anything a good legal document doesn't do. Likewise if you remove the religious element then why not have civil unions?