> If people don't have a right to reasonable private opinions

I question the assumption that this opinion: "gay people ought not be allowed to marry" is a reasonable one. I'll accept common, but I don't agree that it's reasonable.

Also, he does still have the right to that opinion. He just doesn't have the right to express it free of consequences.

Also, he didn't hold that opinion privately. He held it publicly (by donating money to support its enforcement via the law).