Is sex between people with "mental retardation" morally permissible and, if at all, under what conditions? This paper tries to answer this question, but only with regard to sex between biologically mature individuals with mild or moderate mental retardation. First, the concepts of "sexual activity" and "mental retardation" are analysed briefly, which is challenging given the widely divergent and sometimes rather awkward definitions of these concepts. On the basis of this analysis, it is argued that the liberal principle of mutual consent, if taken as a necessary condition of permissible sex, has unacceptable consequences for people with mental retardation. Many forms of sex between them would be morally impermissible, given the fact that their limited powers of practical reasoning will often make valid consent well-nigh impossible. As an alternative to the liberal principle of permissible sex, conditions are specified that include the additional consent of caretakers. If people with mental retardation do not have the capacities of practical deliberation required for valid consent, care providers with mature reasoning powers should act as their substitutes. Finally, some important implications for the moral education of future care professionals are spelled out.