First, let's distinguish between a moral value and a moral obligation. Values, in this technical sense, need not be subject to Kant's categorical imperative. Being a doctor has value, and so does being a librarian, or a teacher. One is not violating the categorical imperative - in Biblical language, "do unto others as you would have them do unto you" - by choosing one vocation as opposed to another.
Kant's technical philosophical definition of the categorical imperative is this: act only in ways that you would will to be universalized.
Now, the reader needs to keep in mind that the argument against same-sex marriage (SSM) is only applicable under the paradigm of Kantian deontology. Nevertheless, the argument adds to an already growing list of reasons to oppose SSM. It is also assumed that adultery is wrong, which is a metaphysically certain consequence of the categorical imperative.
1. One should only act in ways that one would will to be universalized. (Premise, categorical imperative)
2. The universalization of SSM would have disastrous effects. (Premise)
3. Disastrous effects are a violation of the categorical imperative. (Premise)
4. Therefore, SSM should be avoided. (From 1 - 3)
Lest anyone object that SSM is based on a value, and not an obligation, this is demonstrably false. Sexual acts of whatever variety are a matter of moral obligation, and not merely on value-so-defined. Surely the advocate of SSM does not view adultery or hebophilia (sexual attraction to pubescents, roughly from the ages of 11 to 14) as anything less than violations of moral obligations. To make homosexual acts an exception without providing any sufficient reason is to engage in special pleading.
I conclude, then, that SSM is a violation of the categorical imperative.