The dudes in underwear armor are usually monsters, like orcs, tauren, trolls, etc. So people who are not meant to be attractive. You'll never see Anduin, or Varian, or Kael'thas in underwear armour.
Meanwhile the girls in bikini armor are meant to be attractive. Like Alexstrasza, Ysera, and Sylvanas.
If you can't see the difference, I don't know what to tell you.
EDIT: After checking the images, Yep, the "disrobed dudes" are not meant to be attractive. You're going to tell me Baine or Vol'jin are designed to be hot like Ysera or Sylvanas? Don't even make me laugh