I don't think homosexuality can be considered an illness, and certainly not a mental one. I suppose it could be described as a genetic abnormality (in the sense that it is not common enough to be considered normal), but to describe something as an illness suggests that it's harmful, and to be blunt, homosexuality isn't harmful. And in this day and age, concerns about how it interferes with the mating ritual are moot; with population pressures the way they are and the existence of artificial insemination, it's hardly an issue in that regard. Plus, I can't imagine it being especially long before they can do genetic manipulation in order to combine two sperm and generate a zygote from them.