This is not the first, or the second time that he has done so.
Yonmei at the Feminist SF blog rightly takes him to task.
The article is interesting as it effectively dances around the point. The issue of the roots of marriage, the nature of heterosexual relationships and the distinction between a Republic and a democracy are all smoke screens. The issue is not whether GLBT people should be allowed to get married, the issue is whether or not GLBT people should be treated equally in the eyes of the law. As such, all arguments against gay marriage are in fact more or less convoluted arguments in favour of continued discrimination.
In this respect, Card is not doing anything that any of us have not done before when we have argued politics. Because we live in a meaningless universe with no over-arching moral meta-narrative, all ethical discourse is effectively pointless. Much like astrology and theology, ethical systems are closed loops with no truth-makers in the real world. Unlike science, we cannot go out and test our moral theories any more than we can test our theories about how many angels can dance upon the head of a pin. As a result, ethical arguments should not be understood as arguments over facts but rather as a form of diplomatic signalling between world-views. It's saying "I believe X" and "I also believe Y and Z so people who support Y and Z should also be on my side".
For example, when Card says :
He is making common cause with straight married people but, wary that some people cannot have kids, he adds a caveat in order to keep them onside."When a heterosexual couple cannot have children, their faithful marriage still affirms, in the eyes of other people's children, the universality of the pattern of marriage."
Of course, the problem is that by trying to get people on his side to defend against the perils of gay marriage, Card is effectively making it clear that those of us in favour of equal rights are on the other side and as more and more of the SF community either are GLBT or GLBT-friendly or just civil rights friendly, Card is digging himself a nice little hole.
I see no problem in his doing this. We all do it.
What I think is problematic is that Card keeps being given columns in which he can spout his homophobic hogwash. Frankly, he is given these columns because he has a degree of fame due largely to the SF community. Were it not for SF fans, Orson Scott Card would be just another hypocritical fascist with an axe to grind.
I don't generally hold with No Platform policies. I think that they're unhealthy and make martyrs out of people who deserve only to be held up for ridicule and contempt. So I don't think that people should start boycotting conventions or professional bodies that are foolish enough to give him prizes, I think that a much better way of dealing with Orson Scott Card - Homophobic Talking Head is by engaging with him fully and refusing to allow him to talk about anything other than gay marriage.
If he speaks at a convention - Every question should be about gay rights.
If he picks up an award - The body making an award should be submerged with emails either questioning their decision to give a homophobe an award or praising them for making a stand against the homonazis.
In fact, the first person who gets into the Literary Boot Camp and spends the entire week making Orson Scott Card read and comment upon "My first time with Bob" stories will get a case of champagne from me.