"Sex Idiot" is a term popularized by 30 Rock. It refers to a person with whom one has a purely sexual relationship, but who is otherwise an idiot. Broadly speaking, a sex idiot is someone who is sexy, great in bed, but a total moron. One does not exclusively date a sex idiot.
See Also: pokemoning
Person A: Uh, Jake's hot girlfriend just asked me the number for 911.
Person B: Dude, that's not his girlfriend, that's his sex idiot.