A just-released Ninth Circuit opinion has a fascinating debate on this difficult issue. See especially pp. 7264 and 7278-84, which should be quite accessible to laypeople.
I've long been interested in this issue (as a purely academic matter, I assure you). Sex with the mentally retarded is generally treated as statutory rape — just as children below some age are seen as legally incapable of giving meaningful consent to sex (even when they are in fact enthusiastic about engaging in the act), so are the mentally retarded. And that does make a good deal of sense.
But at the same time, if you take this seriously, the mentally retarded would be legally forbidden to have any sex ever in their lives. That's quite a burden, it seems to me, and a much greater one than for children, because it's permanent rather than just temporary.
In practice, I suspect that many mentally retarded people do have sex — some exploitative and likely physically or emotionally harmful to them, and some quite fulfilling and likely beneficial to them — because these laws are quite underenforced. Still, the existence of the laws means that these people's sex lives are entirely on the sufferance of government officials. If the law were completely enforced, the mentally retarded would have no sex at all; and anyone who is engaged even in a mutually good sexual relationship with them is at the mercy of the prosecutor.
What solution can there be for this? And even if the answer is to stick with prosecutorial discretion, how should prosecutors exercise this discretion? Should they basically let parents decide who should be authorized to have sex with the mentally retarded person? Should they do so, but subject to the prosecutor's veto (or some judge's veto) when the parent's actions seem exploitative (for instance, if the parent is essentially pimping the child, for instance letting their drug dealer have sex with the child in exchange for drugs)?
Should there be some judicial override of the parent's veto, as there is for children's abortion rights? Should prosecutors or judges try to distinguish "exploitative" relationships from "beneficial" ones? If so, should they try to avoid their own value judgments (e.g., casual sex bad, committed relationship good, group sex bad, one-on-one sex good, homosexual sex bad, heterosexual sex good)? Or is it permissible for government officials to impose such judgments where control of the mentally retarded is involved, even if it would be impermissible (or at least unwise) to restrict the actions of mentally competent adults on these grounds?
I don't know the answers here (nor am I looking that actively for the answers, since I don't write much on this subject). But I am pretty sure that these are interesting and difficult questions.
UPDATE: Reader Seth Tillman asks whether there may be an exception to these statutory rape laws when the person is married. Laws that restrict sex with underage people often do have an exception for married couples, though of course the marriage laws themselves have age thresholds (and sometimes, I believe, one threshold for marriage with the parent's consent and one for marriage without the parent's consent). Perhaps some states' restrictions on sex with the mentally retarded also have an exception for married couples; I hope to check into this in a few days, when I have a bit more time.
But even if this so, then this just highlights some of the other issues I mentioned. Requiring people to marry before they can have sex is a pretty serious burden on their autonomy -- a burden that we have in practice rejected for competent adults. Should the rule be different for the mentally retarded? Also, presumably people who lack the mental capacity to meaningfully agree to sex would often also lack the mental capacity to meaningfully agree to marriage -- to lifelong sexual fidelity (at least in the absence of divorce, but how does one decide whether a mentally retarded person really consents to seeking a divorce?), to lifelong emotional commitment, and so on. Does it make sense to require them to marry as a condition of having legally permitted, no-fear-of-the-prosecutor-locking-someone-up sex?
And of course there is one sort of sex that marriage won't make legal (except in Massachusetts): Sex between people of the same gender. If you oppose recognizing same-sex marriage but also oppose locking people up for same-sex sexual activity -- and I suspect that many tens of millions of Americans take that view -- then what do you do about mentally retarded adults who have sexual interest in people of the same sex? Do you just say "Sorry, it's not legal for people to have the sexual relationships with you that you want, though it's legal for heterosexual mentally retarded adults (since they can get married)?"