"Kentucky’s marriage laws treat homosexuals and heterosexuals the same and are facially neutral. Men and women, whether heterosexual or homosexual, are free to marry persons of the opposite sex under Kentucky law, and men and women, whether heterosexual or homosexual, cannot marry persons of the same sex under Kentucky law,"I'll be a bit more generous than some of the news outlets that reported on this by noting that the brief is 42 pages so there may or may not be other more rational arguments presented (I didn't take the time to read it). Regardless, it is hard to believe that the argument above could possibly be presented as a serious justification for upholding it's ban on gay marriage.
Imagine a law which states that it is now illegal to practice Christianity and then arguing that it doesn't discriminate against those who are Christian because it also applies to Muslims, Buddhist, and atheist. This sort of specious legal reasoning really drives me crazy. I can't imagine that the Supreme Court will give it any credence.
HuffPo: Kentucky: Our Same-Sex Marriage Ban Isn't Anti-Gay Because It Applies To Straight People, Too