stand behind a female character with the widest shoulders or stand behind any female with fending gear. (which cuts out any additional curves) and all you're left with, is wide shoulders, narrow hips. Thats the PRIMARY method in which almost all people use to judge the persons gender from a silhouette.
Ive never confused a guy for a female (excluding lalas) but i have confused females for males. The ONLY reason I would come close to confusing a male for a female, is because of clothing, but id have to be pretty far away, and not paying full attention. (Of course ive played the game long enough now, that i can tell the race/gender from just animations, w/o having to actually think about it. and similar for general standing poses.) Such as this^ which is a bias, not because of actual structure of the model.
Ive had plenty of FFXIV players think this is crazy, but its pretty easy to prove. Just get a picture of the backside of the characters, and no distinguishable clothes/hair commonly associated with a gender. (just basic Tshirts, etc) Then show it to someone who doesnt play FFXIV/games. You'll see quickly how many ppl assume the gender based on shoulder to hip ratio.
I know the joke is that all the males in FFXIV look "girly", but its always nonsense. They mostly just mean the faces, and hair. And they let their bias alter their opinion of everything else about the characters body. Such as this^. pure bias. Miqote males are more muscular/wide shouldered than the human midlanders. But everyone pretends they are so effeminate, just because of social bias.
Humans use the shoulder to hip ratio for silhouettes, but prefer to use "faces" as a means to determine gender. This is where the bias comes from, since neotonous* features are more typical of youth or females. (*"Cute") And FFXIV and most japanese products give neotonous facial features to both genders.
(Also reminds me of the complaints WoW players had for blood elf males, saying their bodies were shaped like womens. And im like, wuuuut? They have literal comic book super hero bodies, wide shoulder, massive muscles, etc. But since they compared them to the OTHER males in the game, which were more akin to the Hulk in proportions, they somehow thought blood elf males werent muscular/big. (also easily provable with silhouettes, and comparing to comicbook heroes, vs real life athletes with muscular builds.)