I am sorry, what is the problem?
I have worked for both Mencap and for other charities for children with learning disabilities and you have to recognise that people with disabilities aren't all the same, some are more able to be in a loving relationship than others. Many Mencap members are in relationships, some have even got married, some have children. Just because they have a diagnosed need, doesn't mean that they can't have feelings just like any other human being.
Otto seems to have a very mild form of Down's Syndrome, which means he can comprehend what a relationship is. Not all people with Down's can do that, and those who can't need to be protected as vulnerable individuals, however if his mother supports him, why shouldn't we?
I am pretty sure the article was meant to provoke a certain reaction by focusing on the physical side of a relationship, rather than what he actually wants. Having a learning disability can be a lonely place, I can understand the want for affection.
Please don't judge everyone with a learning disability by this article, and understand that just because they aren't 'normal' they can't have 'normal' feelings.