First things first. Read the title carefully! If you are sensitive towards homosexuality or insecure of your favourite superhero sexual preferences please avoid reading! I am not homophobic (at least not anymore), I am just sharing my stance towards some superheroes and how they present themselves. All these points do not apply and are just some stupid thoughts of a geek.
As Clark Kent, he has to keep a low profile and be as ‘Joe the plumber’ as possible. He was only ever in two big relationships in his life, one with Lana Lang and the other with Lois Lane! But there’s simply no way Clarky ever got to third base. I mean, come on let’s face it, the guy would probably literally shoot right through his lady friend. Clark Kent may be somewhat boring but whenever crime roams the streets of Metropolis...well that’s when it gets interesting! The tight rainbow coloured outfit (gotta love those red speedos), his shiny ringlet and his oh so white teeth. I mean as a superhero you probably wanna go more with something that says "badass" like Batman's black outfit (though seriously Bruce the fake abs are so 1983). When Batman shows up people freak out! But did you ever see criminals running away from Superman?... Why would they? His outfit just spreads joy and for crying out loud people first confused him with a bird!! More likely Superman, after ordering the culprits to surrender, would get something along the lines of "Ey, sweetheart parade’s on the other side of town".
Now, if you never got laid and people constantly took a piss on you, it would be perfectly understandable for you to go haywire....but Clarke Kent isn't the slightest bit inconvenienced by these things, thus asserting the conclusion that our beloved hero is not a bird.. not a plane... it's a sexually unreferenced metro.
To put it in a nutshell: Superman dresses up like a metro, fakes sexual interests, and his only true sexual encounter was with himself! Thus the conclusion that Superman is metrosexual with no sexual preferences.