Evidence? Aside from Sheik being called "a man" by Princess Ruto, the official art and in-game appearance being masculine and different from Princess Zelda's body type, plus the eye colors being different?

The official Nintendo's Player Guide also states that Sheik is a male.

Taken from the Zelda wiki:

In the game itself, some of Sheik's grunts seem vaguely masculine. Additionally, Zelda and Sheik do not share physical characteristics, such as Zelda's helplessness when trapped by a ring of fire in Ganon's collapsing castle; this is a jump that she would presumably be able to make as Sheik, implying an actual transformation or magical augmentation as opposed to a mere clothing difference. When Princess Ruto describes Sheik to Link, she refers to him as a "nice young man", and in the original Japanese version, Sheik introduces him/herself using boku, a pronoun mostly used by young males Manga has Sheik as a male [...].

Not to mention the manga stating Sheik is a male, although this is a non-canonical source. But the point is there's enough evidence to suggest gender-bending here.

In Persona 4 characters called Naoto a boy before they found out she was a girl so that's not really a strong case neither is quoting a fan speculated source. The Zelda/Sheik thing is something that's been in countless animes and various games, Fire Emblem even had a lite go at it in Awakening.

I'm not stating that Sheik is ultimately a male, I'm saying that there's enough evidence to suggest the possibility of gender-bending.

Yeah, that basically sums up my thoughts. We don't really know about it. There's evidence suggesting genre-bending. But nothing definitive. And I guess we'll know in the E3 about that. I think that's a possibility which is not so remote.

Point is your evidence is fan speculation and speculation that just seems to be looking for something that isn't there and is already explained by basic events, Ganondorf conquered Hyrule by usurping the thrown. Impa flees with Zelda and trains her in the way of the Sheikah, knowing that operating as Zelda would attract Ganondorf's attention due to her possessing one piece of the Tri-Force she disguises herself to the point of using masculine attire to not attract too much of Ganondorf's attention, what's one Sheikah going to do against him after all? Only another holder of the Tri-force can deal with him and he knows who holds the other pieces.

Considering Zelda's magic, the fact that she could use it to swap bodies isn't far-fetched at all. That totally defeats any attempt of Ganondorf finding out where Zelda went. Because fan speculation or not, Sheik and Zelda don't have the same atributes.

Again male attire and a bit of magic can give the impression of different attributes, another incarnation of Zelda swapping bodies doesn't mean this one can, also if you look at Sheik in Smash the voice is androgynous like a woman putting on a deep voice which highlights to me it's more a disguise.

I'm not talking about Sheik in Smash, which Nintendo has been making more affeminated with each passing entry. Sheik's grunts in OoT are somewhat masculine.

Although the fact that Sheik and Zelda are now different characters in Smash is telling, no?

That was done because of limitations of the 3DS hardware.

The grunts can simply be her trying to hide her voice, if Nintendo see her as more affeminate than that speaks a lot more about the character than speculation, even in HW Sheik is affeminate highlight the character was always female in disguise which in a lot of Japanese has been a popular theme.