I wish the article linked in the first post had itself actually linked to the newspaper article it was referring to -- or even the court record.
I don't know how it works everywhere else, but AFAIK in the UK, if one or both parties are legally unable to consent to sex because they are below the age of consent (= 16 in the UK), but they go ahead and have 'consensual' sex anyway (as teenagers everywhere have done since time immemorial) then technically the older partner (or both) may be charged. In practice this usually doesn't happen, provided that both parties are also legally minors (age of majority = 18 in the UK), and/or the age gap isn't significant (informally referred to as 'Romeo+Juliet laws'). So e.g. a case involving two 15 yr-olds would usually be ignored (unless it involved coercion and/or resulted in pregancy), a 14+16 yr old might be considered borderline charge-able on principle (depending on e.g. whether the actual age gap was 1 yr + 1 day, or closer to 3 yr - 1 day); and a 17 yr old would almost certainly face charges (as a young offender) if they had sex with a 13 yr old (and rightly so, in my view).
Extending that idea of competency-to-consent (or lack thereof) to the mentally disabled, I think it would be reasonable to argue that, provided both partners are of roughly the same mental competency (mental age, if you like), and technically adult by birthdate (even if they have a legal caretaker/ guardian), that it should not be illegal -- or at least, not generally punishable -- for them to have sex with each other. If the desire is there, then they clearly grasp the 'value' (for want of a better word) of extending their relationship into the sexual. So yes, make sure they understand the risks as well, and how to look after themselves -- even if that doesn't go any further than their learning some basic rules like "If you don't like [something], you're allowed to say no, and the other person must stop (and vice versa)" and "Always use a condom".
But if it's a question of an able-minded person wishing to have sex with a mentally non-competent person (or vica versa, and the nominal 'adult' agrees), I would consider that an abuse (or exploitation) of trust in an unequal relationship, similar to how bosses coercing employees into providing sexual services, or teachers having sex with students (especially if under 18 years old), would/should be considered abusive. And if it happens, the abuser should likewise be charged.