Outline a story that features a strong male protagonist and a damsel in distress. The male protagonist should be hesitant, but gets involved when it becomes clear he's the woman's only hope. (There are variations on this theme: a male protagonist who gets involved when there's something in it for him other than the lady, a female protagonist who has to rescue a male, etc.)
Adhere to the traditions of the Western novel. Keep facts accurate, use Western terminology, and create settings that fit the genre. Don't put women in sexy outfits that didn't exist in the 1800s. Writing an adult western with some erotic content doesn't mean being inaccurate.
Plot a story that doesn't rely on the erotic elements. Erotic scenes should support the story, not be the focus. The main story should still follow the traditions of the Western novel.