So one day while drinking, the topic of superheroes came up. (Surprise!) In discussing whatever aspects we leaped and bound through, we realized something:
There’s a Spider-Woman, but no Superwoman or Batwoman!
Well, since that day, I’ve done some investigating, and the others have existed. Hence, I’m changing my postulate to this:
No major character spin-off with ‘woman’ in her title has ever had her own comic except for Spider-Woman!
But what about Wonder Woman, nerds you cry, or Catwoman!? They aren’t spin-offs based on some male version (even though there is a Wonder Man over at Marvel, and who the hell was Catman?)…
Even Hawkwoman and Invisible Woman were originally Hawkgirl and Invisible Girl. (Okay, so Hawkgirl and Hawkwoman are different characters, but latter hasn’t had her own comic line like the former.)
Anyshehulk, in the Marvel Universe, Spider-Man has nothing to do with Spider-Woman’s origin, whereas the Superwoman and Batwoman incarnations had ties to their male counterparts:
So what makes Spider-Woman so unique? Compared to the above, Jessica Drew has almost always been Spider-Woman. And she came before any of the other Spider-Man counterparts, too:

Spider-Girl is Spidey’s daughter, May; Spider-Boy is a mixture of Spider-Man and Superboy in a combined comic event (so in other words, not real, figuratively speaking).
Supergirl and Superboy have always been Supes‘ cousin or him as a kid (or later, a clone):
And as for Batgirl and Bat Boy…
SIDENOTE: For more on sexism in comics, check out Women in Refrigerators… this just turned into a PSA.