edit: Or wait.... you don't know what that means, do you? You think classification and discrimination are the same thing in this context? No wonder you've been confused. Sorry, man. I should have figured this out sooner.
Classification is a term used describing how the law actually is functioning, and on what basis. Like the Texas Constitution example I posted earlier, it defines marriage as between one man and one woman. This is why I use the term "gender-based classification." The actual legal language refers to gender. The discrimination, of course, is against homosexuals.