> For example, homosexuality is rare...decreases the chance of having direct descendantsThe gene for male homosexuality is actually propagated via over-fertility in females with the gene (i.e. gay guys' sisters have more children to compensate). So, reduced direct descendants yes, but reduced passing on of genes no.
> Now switch homosexuality for some far less socially acceptable...How does things change?
That involves moving out of the realm of consenting individuals. That is what changes.
I agree that there is a point at which abnormality becomes illness - my (draft) definition is drawn when the abnormality causes pain (especially chronic pain), or early death.
That applies to most old-school illnesses, but not to homosexuality, or autism, or some other disabilities like being short a leg. Specifically on being able to reproduce, I wouldn't say that a mule was disabled, even though it has an odd number of chromosomes and is therefore sterile. Same with the bees; although hive-based species are always going to seem weird to mammals like us.
Other disabilities, like arthritis, do meet the illness threshold because they do cause chronic pain. With this definition, those with disabilities may ask for additional equipment, support, or perhaps even elective interventions like TMS, but only those with illnesses need medical interventions (whether to mitigate or cure).