Disney, there are a number of things that are absolutely arguable as legitimate state interests that can be argued with regards to marriage. Creating stable families, encouraging procreation, advocating monogramy in the population, better regulating ownership rights in joint couplings, and even for lack of a more eloquent explanation streamlining the aiblity to set a singular individual as your prime designee in most scenario's.
While various people may disagree with how much of a legitimate state interest those things are, its just not accurate to suggest that there is an issue in coming up with any legitimate government interest the government has in regards to marriage. Indeed, it has been decided...through upholding the governments role in marriage to date...that the government DOES have legitimate interest in regulating marriage.
Same thing for
Where the EPC comes into affect in terms of "gay marriage", and through use of the term you're clearly talking about the class "homosexuals" rather than gender, is not singularly whether or not there's a legitimate state interest at play in the particular instance but also whether or not discriminating against the class in question is "rationally related" to that interest.
It is that rationally related portion of things where potential problem for those against it come to a headway. However, there's still a large question if even something as nebulous as maintaining a traditional culture is enough to qualify as "legitimate" but not "compelling" (Which would be what was needed in Loving).
Considering that homosexuality, at this point, is a classification that falls under minimum scrutiny and the very broad leeway that it bestows upon judges in determining what "legitimate" and "raitonal" means, I don't believe the issue is nearly the slam dunk...when viewed from the mindset of "gay marriage"...as your opinion presents it.