That's the missing standard. There is no longer any reason to explicitly state that it is between 2 adults not related to each other. To do so would be seen as 100% arbitrary.
They threw out religion. They threw out existing law. They threw out popular sentiment. Now, there is no reason to define it at all. Marriage is now whatever anyone wants it to be.
You're right! I don't want a Constitutional Amendment added over this issue. I hate to say this but I would rather have gay marriage than an amendment in the Constitution defining marriage.