I’ve wrestled with this issue myself. Ultimately, regardless of whether sexual orientation is biological in origin (best evidence indicates that at least partly it is) or comparably socially or genetically to the texture of one’s hair*, or for that matter even if it’s 100% a choice and all gay people secretly choose to be gay (which I don’t even think most people in Kabul would believe) it doesn’t in any way change my belief in the rights of gay men and women to marry. As another poster commented, nobody is genetically Pentecostal or Eastern Orthodox or a Voodoo practitioner, yet religious freedom is granted without question.
As for blacks v. orientation in terms of “civil rights groupings”- the similarities definitely seem to outweigh the differences. Neither race nor orientation can be convincingly argued (i.e. using studies based on scientific evidence properly gathered and analyzed by qualified persons) to be a choice. Both represent a minority of the population in western cultures (although black people have Africa to claim as a homeland and gays have reclaimed urban areas with warehouse lofts). Both groups The subject of prejudice and have a history of persecution from others that is based on learned belief systems rather than on empirical merit. Both have had their marital ranges limited by laws based on non-scientific religious and political beliefs. Both have to endure stereotypes and really bad cardboard television portrayals (often by David Alan Grier in both cases).
(Certainly some black people would agree with the comparison judging by the voting records on whether state constitutions should be amended to ban gay marriages; consistently, the majority of black legislators have voted on the “gay side”.)
As to whether it’s easier to be black or be gay in modern times (assuming you’re not both, in which case you really have some bitching rights), that’s a real debate. OT1H, gays usually have a much easier time “passing” as the majority when convenient and aren’t that likely to be pulled over by cops for no apparent reason. OTOH, I believe it was E. Lynn Harris (the gay African-American author) who said “At least you never have to worry about how to tell your mother that you’re black.”
*Hair texture, not skin color, is the measurement of “race”. I believe I’m correct when I say that DNA testing will not reveal a person’s “race” (or, of course, their sexual orientation) with current technology because on the genetic level there’s no real difference (and besides which, we’re all muts).