A demon is presumed to mainifest itself in whichever form is most likely to attract the person whose soul the demon is trying to steal.
Therefore I assume that a succubus, a female form, would be used to attract a male or a lesbian, and an incubus, a male form, would be used for heterosexual women and homosexual men.
Whichever appears, please remember that the appearance is only that. The reality is a demon from the depths of hell and its sexual endurance is far beyond human capacity (even with Viagra!).
