Stop saying it. Please. Nearly every time gay marriage comes up, someone proposes we just "drop" the issue (or actively campaign against it), because it supposedly doesn't play well with African-Americans, who are thought to be highly religious and thus anti-gay, although highly Democratic. There is a justified concern about Republicans using such diversionary tactics to split the Democratic base, but our real problem is not that black people oppose gay marriage, it is that white people oppose gay marriage.
Statistics in the extended entry...
Last November, several states with large numbers of black people voted to amend their constitutions to ban either gay marriage, civil unions, or any kind of domestic partnerships. They were:
Arkansas: White voters approved it with 77%, black voters with 66%.
Georgia: White voters approved it with 76%, black voters with 80%.
Kentucky: White voters approved it with 76%, black voters with 70%.
Michigan: White voters approved it with 60%, black voters with 59%. (Note: Latino voters barely approved it with 51%.)
Mississippi: White voters approved it with 89%, black voters with 77%.
Ohio: White voters approved it with 63%, black voters with 61%. (Note: Only 44% of Latinos voted for it).
Oklahoma: White voters approved it with 76%, black voters with 74%.
In only one state did a higher percentage of black people vote for these referendums than white people. In three states a significantly lower percentage of black people voted for the referendums than did white people. And in the two states with sizable Latino populations, even lower percentages of them voted for the referendums (thus refuting the "Latinos are too Catholic and anti-gay" talking point as well).
The source is CNN's exit polling.