How are disabled people treated in a free use world? I can't imagine that everyone would be up to it

Well, in a world with perfect birth control and the eradication of every single STD, there are probably very few disabled people, as those things would have seen cures as well.

However, for those that exist, they have just as much right to use others in the FreeUse societies as anyone else does and just as much right to be used. So, since there’s no taboo about “kinks” in FreeUse settings, all the people who are into that are going to feel free to go after what they want. Basically, everyone is getting what they like best and for a section of the populace, that’s… amputees and paraplegics.  Since there’s no social shaming of sex at all, it’s just another set of preferences. Like pizza toppings.