Last I checked there are more than two sexes (e.g. intersex) and so "opposite sex" means almost as little as "opposite gender" does. Yay yet another idiotic Writers' Block question (punctuation of title corrected by me because there are surely more than one writer).
The question I answered here, in case they change it (which they have done before), was, "You wake up as a member of the opposite sex. What is the very first thing you do?"