This is what Maureen Milligan and Alfred Neufeldt (1) call the 'myth of asexuality' applied to disabled people. They associate it with two common assumptions about disabled people:
1. that actual or presumed sexual dysfunction limits gratification to the point that sexual needs are deemed absent or subjugated, and
2. that intellectually disabled or mentally disordered people lack the judgement to engage in responsible sexual relationships.
(The second one seems a bit controversial, but I doubt they'd universally apply it to mentally disabled people.)
As the news article points out, these assumptions aren't necessarily true at all, but they're reproduced in media all the time.
"Inclusion does not just mean building a ramp."
(1) Milligan, Maureen S., and Alfred H. Neufeldt
2001 'The Myth of Asexuality: A Survey of Social and Empirical Evidence'. Sexuality and Disability 19 (2):91-109.