I'm bisexual, don't believe in marriage as an institution outright (so gay marriage is fine to me, I think it's just as pointless as regular marriage), and read many works of Orson Scott Card, such as Ender's Game and its sequels, and a number of his other works such as Songmaster (which has homosexual themes and even sexual conduct; even if he considered such to be a scene marking the start of a destructive path, he is not hostile to homosexuality).
In short, this is an issue being blown out of proportion. To summarise:
OSC is entitled to his opinion; that homosexuals should not be allowed to marry, as that is a violation of the religious tenets that support marriage. He believes it is a violation of the institution.
Christian Nutt wants to boycott Shadow Complex as it was written partially by OSC. He believes that purchasing a work made by someone who publicly denounces this is condoning them, or at the very least supporting their view. He's entitled to this opinion. He's also entitled to write essays on it and post it to the internet.
I believe they're both wrong. I take the controversial stance that marriage is an outdated and irrelevant institution, its connection to government and tax policies should be absolutely scrapped (as couples usually receive tax benefits over single people), and it thus becomes entirely a church-by-church basis as to whether homosexual marriages are recognised or not. I'm entitled to this opinion, regardless of how outlandish it may seem in light of anyone else's views.
I won't be buying Shadow Complex myself, but that's just because I don't have an Xbox 360 and hadn't actually heard of it until now!