Slate has a very interesting article on the subject, and last year had another, focused more specifically on the legal issues. The theory that people with sufficient mental disabilities lack the capacity to consent, and therefore sex with them should be illegal, is sensible and probably usually right. But the consequence, when the disability is irreversible, is that they are doomed to live the rest of their lives without sex (or with sex that makes their partners criminals, and makes those who facilitate the sex -- for instance, relatives or nursing home operators -- into criminals as well).
My highly tentative sense is that giving guardians the right to consent on the person's behalf is probably a reasonable approach. We rightly don't generally let parents do this for children, partly because for children we're telling them just to wait a few years, not to remain abstinent for the rest of their lives, and partly because we fear that allowing such parental consent will lead to a considerable number of parents prostituting their children (a tiny fraction, I'm sure, but a considerable number). But these reasons don't apply to the elderly permanently disabled, and at least the first reason doesn't apply to the younger adult permanently disabled. (Pregnancy risks are also not a problem for the elderly permanently disabled.)
In any case, though, it strikes me as an interesting and important subject, and I'm glad Slate is covering it.