It depends on the degree of "retardation." I'm not sure of the legal threshold in the USA but I'm sure if it dips below a certain level the model would be legally deemed to be incapable of contract much less consent to sex acts. In that situation, criminal prosecution for rape (sex with a person who didn't or CAN'T legally consent) is a possibility. See
http://www.rapecrisisonline.com/Myths.htm Just my 2 cents.