There is very little actual difference between people of different races. There’s a couple exceptions, like genetic diseases that affect some races more, and the fact that people with darker skin need more sunlight for Vitamin D and sunburn less easily, and some minor differences on average in body shape, but pretty much, race doesn’t affect what a person is capable of and how a person can function in society.
There is a somewhat significant difference between people of different sexes. About half the population has a chance of becoming pregnant at some point in their lifetime; about half the population does not. However, with modern birth control, this difference has been ameliorated, and the sexes can function very similarly. And of course, anyone of any sex can function neutrally if celibate.
There is a fairly small but real material difference between orientations. Gay and non-attracted people are much less likely to have unplanned children than straight or multi-attracted people, and less likely to have biological children in general. Orientations have no material effect outside of reproduction, and in other areas of life people with different orientations can function identically.
The differences between races are vastly exaggerated by society, and the extreme gap in social roles between them is caused by racism. There is no inherent need for people of different races to be treated differently. The medical field may draw technical distinctions that line up somewhat with the current conception of race, but non-specialists have no material reason to care.
The differences between sexes are vastly exaggerated by society, and the extreme gap in social roles between them is caused by sexism. There is no inherent need for people of different sexes to be treated differently except as regards pregnancy. In most ways, a person who can become pregnant but is not currently pregnant is not any different from someone who cannot; there is essentially no difference at all between a person who can theoretically become pregnant but is on birth control or celibate and someone who cannot under any circumstances.
The differences between orientations are vastly exaggerated by society, and the extreme gap in social roles between them is caused by heterosexism. There is no inherent need for people of different orientations to be treated differently, although gay and non-attracted people tend to be benefited more than others by improvements in adoption services.
Disability is not like that. The differences between having and not having a particular disability are real, material, and substantial. Disabled people are kept down by society in many ways, but disabled people are also, even without that, kept down by the fact of their disabilities.
Society treats disabled people differently, and this is not always a bad thing. Some ways in which disabled people are treated differently are bad–mockery, social ostracism, and outright hate crimes, to name a few. But allowing disabled students access to learning materials that their disability does not prevent them from using is also a way of treating them differently, and is not bad.
People should be treated equally regardless of race, sex, and orientation. I firmly believe this, and it’s a goal that I have some idea how to approach: identify instances where I treat people differently based on these factors and make an effort to stop; analyze cultural factors that lead to discrimination and resist them in my everyday life; name policies that cause or enable unequal treatment and work with other activists to change them.
The same statement as applied to disability is untrue. It is untrue logically, factually, and ethically. Ableism is just as real as racism and sexism and heterosexism, and I have no idea how to fight it.