To those who believes that legal marriage should obviously apply to any two people who choose to be married and that laws that don't apply equally to same-sex couples as to opposite-sex couples are discriminatory, anything that prevents a same-sex couple from marrying is the equivalent of anti-miscegenation laws. Contrary positions would be the equivalent of racism. As African Americans have been sorely hurt in the US by racism for hundreds of years, they should be natural allies - and failing to agree is hypocritical.
Those who believe that by definition marriage is between exactly one man and exactly one woman (at any one time) have a fundamentally different view. To begin with, sex (from the genetic basis to physical characteristics) is a well accepted distinction between people. By contrast, race and ethnicity are arbitrary constructs. Anti-miscegenation laws had to be enacted because it was quite obvious that people of different races could marry; until very recently, there was no effort to create anti-same-sex-marriage laws because, well, the very definition of marriage didn't allow for that in a fairly fundamental way.
Of course, there are some parallels. Socially, couples that cross different ethnic groups are more accepted than at some earlier times in US history (though there are still groups within society who disapprove of these) - the same is true of same-sex couples. While most religions recognize inter-ethnic marriages, some don't; one might argue the opposite for same-sex marriages. However, it is neither a well justified assumption that African Americans must be in favor of same-sex marriages, nor is it good politics to take people to task for a betrayal that is only in the mind of the beholder.