A bad argument against bad arguments is a bad argument

On the basis of having assigned numerous papers to his college students on the issue of gay marriage and, not having encountering good arguments against it, James Hanley at Positive Liberty has concluded that there are no good policy arguments against it.

First, the evidence:

Over the last several years, I have assigned the question of same-sex marriage to my American Government students three times. I never ask them to write directly about whether they support SSM, but phrase it as a question of federalism–should the issue of SSM be left to the states or nationalized via a constitutional amendment? Note that the latter option allows for either constitutionalizing a right to SSM or constitutionalizing a ban on it. Students need not allow their personal views to slip into their paper (indeed the ideal paper answers the question solely on the basis of federal vs. national powers, without recourse to the rightness or wrongness of SSM), yet unsurprisingly, nearly every student makes their views on the issue clear. Having now read over 100 student papers on SSM, I can state with certainty that I’ve yet to hear a good justification for banning it.

Then the conclusion:

It’s an intriguing thing to discover that there are no good arguments for a particular public policy. I’m accustomed to thinking there are reasonable and reasoned arguments on each side of nearly every policy issue ... Should we treat gays differently than straights? Sorry, all the best efforts at justifying treating unfavored groups differently have already been tried out and discredited.

The argument seems to go like this: College students are uniformly bad reasoners, and on the basis of the uniformly bad reasoning I have experienced, there has been uniformly bad reasoning in support of the position that gay marriage is a bad idea. Therefore gay marriage is not a bad idea. Quod Est Demonstrandum.

We're wondering what other bad positions we will have to accept because today's college students cannot articulate good arguments against them. But perhaps there is more to his argument:

To those who would cling to discrimination, I say, you’ve lost intellectually. Indisputably lost. You’ve had plenty of time to come up with good arguments in support of the status quo, so if any actually were available I would have heard them by now. But in the last five years, the time during which I’ve been actively thinking and reading about this issue, I’ve yet to hear one argument that has given me pause, even for a moment. And so my patience with the other side in this argument has run out, which means, of course, that I cannot ever assign this topic again. But it was a good exercise while it lasted.

Does this mean he's basing his argument on more than just the papers of his students? If so, then why so much emphasis in his post on his student's bad reasoning--and why only a vague reference to any arguments outside this context? Never mind that he has not proven that laws that don't accommodate same-sex marriage constitute "discrimination": how does citing bad arguments for an opponent's position bolster your own? Don't we routinely question the strength of a team with a weak schedule?

If you're going to be serious about trying to establish that the case against same-sex marriage is weak, don't you have some obligation to cite something better than the papers of your rationally incompetent college students? In fact, I'm wondering what he would think about a student paper written with reasoning as bad as that contained in his post.

I have never encountered a convincing argument in favor of same-sex marriage. And Hanley's post certainly does nothing to improve the situation.