Well, again I haven't even yet offered a comprehensive defense of marriage as I think it should be. To respond negatively however (that is, refraining from producing forth a positive case for my position in the meantime), he hasn't provided any reason to think that marriage should exist for the purpose he asserts it does. Matters of public policy as important as marriage, for example, all exist for some purpose or other or else they wouldn't exist at all. Marriage, thus, exists for some purpose or other, or else it wouldn't exist at all (consider: would anyone have ever thought instituting marriage if the reason for its instituting was "there's no reason to institute it"?). This purpose, however, must be of compelling interest to the state and public good (it can't be a trivial purpose, like, say, "because we felt like it," etc.). Generally, supporters of same-sex marriage argue that marriage ought to exist to recognize people who are "lovingly committed" to one another. But think about whether this purpose is of compelling interest to the state to confer benefits and to the public good at large. Thus understood, this serves no public compelling purpose whatsoever, for why should the state care that, say, Bob has fuzzy feelings for Susan, or that Ted has fuzzy feelings for John? Moreover, why should it be the public's business if, say, Bob has fuzzy feelings for Susan or that Ted has fuzzy feelings for John? These are, of course, just rhetorical questions. For what the supporter of same-sex marriage is really asking for is nothing dissimilar to a government registry of friendships and the only reason they seem to be able to give for the adopting of this is something to the effect of "because it will make people feel good." But such childish reason is of no compelling interest whatsoever to the government of the public good, who would presumably be funding the useless benefits of such pointless enterprise vis-a-vis government appropriation of private funds (i.e. taxation).
Indeed, if marriage really just is a glorified government registry of friendships, as supporters of same-sex marriage seem to think, then the most reasonable position would be that of the contemporary libertarian who constantly gripes on his FaceBook that "marriage should, like, not be a government matter, man -- Hail Ayn Rand!" (there is my feeble attempt at comedic relief). But, of course, I don't agree with the antecedent of that conditional, i.e. I don't think that marriage is just a government registry of friendship or just people being "lovingly committed" to one another; I think that marriage exists to unite mothers and fathers to their children and to one another and to the children that may come from their union, upon whose stability the children depend. And, moreover, I think this purpose is not just of some compelling interest to the state and the public good; I think this is of ultimate importance to the state and public good.