Because they are manly men who do not put their penises in other men. IE, massive degrees of homophobia from the corporate masters and the fanboys, not to mention that you'd get the inevitable pederasty corrolary (OMG, if Steve's dicking Tony, that means he must have abused the underage Bucky!).
Steve/Tony makes perfect sense, because after all America's been in bed with the Military/Industrial complex for ages...
But yes, Steve/Tony works in the same way that you can buy either Dick or Dinah as Bab's exception, depending on which way you have her, um, roll (or Mystique/Destiny for another Marvel example; I don't think Raven's ever been shown to be with another woman apart from Irene, as all her other canon relationships have been with men -- Sabertooth, Wolverine, Forge, Azrael if you accept that crap).