I posted something about this in ronebofh's journal, but it's been getting a lot of attention, so:

Did California's Proposition 8 pass because of increased black turnout in the state?  Barack Obama's Presidential campaign this year dramatically increased the level of black voter participation over its usual level, but exit polls indicated that black voters favored Proposition 8 by a 70-30 margin (whites opposed it by 49-51).  Obama himself is on record as opposing same-sex marriage, and has taken some heat for expressing only very lukewarm opposition to Prop 8 and only in the final days of the campaign.  So white liberals have begun asking: did Prop 8 pass because Obama incidentally drew homophobic voters to the polls?

CNN posted exit polls that tell some of the story: http://www.cnn.com/ELECTION/2008/results/polls/#CAI01p1.  We have to assume that the exit polls are reasonably representative of how the general population actually voted.  That's not necessarily a safe assumption, but since people are already saying that black voters votes 70% in favor of Prop 8, it's clear that we've already accepted that premise.

Now, I'm no Nate Silver, but I don't think this question is that complicated:

10,357,002 votes cast on Prop 8.
5,425,000 were yes votes.

63% of all votes (6,525,000) came from whites.
49% of those (3,197,000) were cast in favor of Prop 8.
That's 31% of the total of all votes cast.

10% of votes (1,035,000) came from blacks.
70% of those (725,000) were cast in favor of Prop 8.
That's 7% of the total of all votes cast.

18% of votes (1,864,000) came from Latinos.
53% of those (988,000) were cast in favor of Prop 8.
That's 10% of the total of all votes cast.

So of all 10 million votes cast in California:
31% of them were white voters who voted "yes" on 8.
7% of them were black voters who voted "yes" on 8.
10% of them were from Latino voters who voted "yes" on 8.

So what?

So:

1. Black voters didn't pass Prop 8.  In terms of the percentage of the "yes" votes cast on the ballot measure and in the absolute number of votes, they were dwarfed both by white and Latino voters who voted for it.

2. Increased black turnout for Obama didn't pass Prop 8, either.  Black voters usually make up 6.7% of the electorate in California.  If Obama's campaign drew 300,000 more voters to the polls than usual, and 70% of those people voted for Proposition 8, that's only about 200,000 votes.  Proposition 8 passed by half a million.  It would have passed even if the black vote was at its usual non-Obama levels.

And, at the risk of sounding like the Monday morning quarterback:

3. Same-sex marriage advocates failed at outreach.  We knew that black turnout in this election was going to be extremely high, and that urban black voters have not tended to favor gay rights issues in the past.  In the end, 70% of black citizens voted to kill gay marriage.  We had an opportunity to engage in a more aggressive dialogue there and we blew it.  Not that we didn't try.  But it wasn't enough.

4. Same-sex marriage advocates failed at organizing, period.  White voters did vote against Proposition 8, but only by a razor-thin margin -- 49-51.  We should have been able to do much better than that.  Gay advocates got complacent with our early polls showing a landslide against the amendment, and we let the Mormons outflank us.

5. Dan Savage needs to get over himself, girlfriend.