Sexlexia is a "very sexy learning disability." Its existence is brought into question by the fact that Zapp Brannigan is the only known sufferer.
Leela: You know, Zapp, someone ought to teach you a lesson.
Zapp: If it's a lesson in love, watch out; I suffer from a very sexy learning disability. What do I call it, Kif?
Kif: [sighs] "Sex-lexia".