Perhaps it began with Wonder Woman as portrayed on TV by Lynda Carter. Or it may go back to the original comics versions of Supergirl, Wonder Woman and Mary Marvel. Whatever the case, erotic superheroine fastasy appeals to men who fantasize about making love with a superheroine as opposed to, say, a Playboy centerfold, an actress or a pop music star.
The fantasy even appeals to some women. "Men want them, women want to be them," is how Velvet Belle Tree puts it. It was only a few decades ago, after all, that women were expected to be weak and submissive, and even today they have to worry about rape and sexual harassment; why shouldn't they want to imagine being superhumanly powerful and alluring -- fearing no evil man, yet sexually irresistible to any good men (Or, if they swing that way, good women) they fancy? Still, it's primarily a male fantasy, perhaps the latter-day equivalent (if there ever was such) of an ancient Greek fantasizing about sex with Aphrodite herself as opposed to a temple prostitute.
There are common fetish elements in erotic superheroine fiction, all centered on strength and invulnerability. Just as with Superman,bullets bounce off superwomen's chests  (and even more private parts), but to them the bullets (ditto the rays from sf energy weapons) feel like tickles or caresses and actually turn them on. They can bend steel easily, and lift heavy weights -- cars and, in the most extreme examples, even battleships. They can bathe in fire, lava and molten steel without harm, and even with pleasure. And, of course, they are exhibitionists who enjoy showing off their strength and invulnerability to men, often for purposes of seduction. Some writers and their fans like their heroines to be ultra-muscular and show off their biceps and abs as well as their breasts and vaginas. Others prefer the toned look of female athletes or ballerinas.
Why should superheroines be interested in ordinary men at all -- who knows? But the fantasy ignores that. The male reader can imagine that this incredible creature, this goddess who doesn't need a man for protecton or support, who can't be raped or intimidated, actually WANTS him to make wild and passionate love with her, that (implausibly, except for the gold weakening device in Aurora Universe fiction) there is somehow no physical impediment to intimacy (the other side of the Man-of-Steel, Woman-of-Kleenex coin), so that man and superwoman can enjoy orgasmic bliss together.
One of the first erotic superheroine stories was Ann Douglas' "The Erotic Adventures of Supergirl" (Her Supergirl swung both ways.). Doubtless the most extensive archive of erotic superheroine fiction (plus photo manips) is Xtreme Strength, but there are also non-AU erotic superheroine stories at Julie of Velor and Infinity Bridge. Subscription websites such as MandiSteele.com Adarasteele.com, heavenlyheroines.com and supersexyheroines.com offer photo layouts and photo stories about costumed superheroines. Altawoman is a subscription site devoted to poser-created comics about its heroine. Projectsuperwoman.com markets short superheroine videos, and other sites have begun producing videos (Kelly Johnston, proprietor of the Steele sites, brought out two full-length videos, GIRL OF STEELE  and THE AWAKENING). While there isn't any sex at these photo and video sites, except for Altawoman. the costumed models and the fetish elements are turn-ons for fans of that sort of thing.
Two variations of erotic superheroine fiction are Peril and Femdom. In Peril fiction and photo comics (There seem to be dozens of websites like Superheroines Demise devoted to the latter), the superheroines are dosed with kryptonite or its equivalent and subjected to abuse. In Femdom, nasty superwomen kill and maim ordinary men instead of offering sexual favors; the most prominent practitioner calls himself Conceptfan.