Now I'll try and be sensible before you get accused of heinous crimes by the Guardianistas.
It could be argued that homosexuality is a genetic trait that would not normally propegate, therefore could be seen as a genetic dead end, a mutation path that would generally in nature die out, so in that sense I see what you are getting at but I wouldn't call it a disability. It is accepted that mankind has ensured survival of genetic faults that would in nature not be tolerated, for example, natural selection has pretty much weeded out faulty sight in the animal kingdom, mankind has changed the rules so that being short sighted no longer ensures being eated before a perfectly sighted colleague in the same species. Would you say being short/long sighted is a disability? Perhaps.