Black culture despises homosexuality. In my state of Ohio, we had a Constitutional Amendment on the ballot to ban gay marriage. It passed with 68% of the vote total and 90% of the black vote.
Oh, did I mention? In that same election, GW Bush got 18% of the black vote. If not for that, it's likely Kerry would have won the presidency. Bush won Ohio by a total of just over 100,000 votes.
When issues are separated, as in the amendment you mentioned, yes, blacks will vote according to conviction. But when it comes to voting Democrat vs Republican, convictions appear to carry little weight. It is the same with abortion. Blacks have consistently been surveyed to be more pro-life than whites. But blacks consistently vote for candidates who are pro-abortion over candidates who are pro-life. When it comes to abortion, you can't get more extreme than Obama, who has totally supported late term and partial birth abortions. But his stand on abortion doesn't matter to the average black voter, even good church-going Christians.