The prevalence of homosexuality is probably way too high to be a real mental illness (compared to schizophrenia for example).
Sexuality as a gradient makes the most sense, especially when the ratio of males to females is heavily skewed.
When then it is a mental illness by degrees. Any way you try to frame it - the less attracted to the opposite sex one is (especially in the animal world where decisions are made by instincts) the less likely they are to reproduce. Since there is a continuum, it's not a death sentence to that genetic line, the specimen may still reproduce, but in Darwinian terms quite a deficiency, wouldn't you say?
Perhaps a more important question is: why is it so prevalent? Why would nature create so many individuals that are so unfit to reproduce?
Sexuality as a gradient makes the most sense, especially when the ratio of males to females is heavily skewed.
On second thought, the second half doesn't make that much sense to me. If there are too few females, how does turning gay help the males reproduce? I could guess that it is an outlet for their need to screw something, but that doesn't appear to be the case in humans, human homosexuality being the real issue here.