there are characters pre 1970 that are written as bi and either have basis in reality if they're historical figures, or basis in their characterization. marquis de sade, dorian grey, oscar wilde, caligula, so on and so forth. bisexuality and homosexuality has existed for as long as humanity has. soldiers constantly screwed one another while away at battle and it was just considered the norm back in ye olden roman days and the like. and that's reality. earlier chars can and have been written as bi/gay, ala dorian grey.
but the idea of 'bi until proven otherwise' only applies if sexuality/interest has never come up. in this particular instance, steve rogers has so far only shown romantic leanings towards women. the argument then shifts to 'well, just because he's never said it...', which isn't necessarily untrue, but is really just a way to grasp at straws to try to make something more legitimate than it is. play him as bi. but don't expect people to agree that it's canon. same as any other fandom where one specific gender has been preferred but 'THEY NEVER SAID OTHERWISE' is the standing argument.
Re: da
but the idea of 'bi until proven otherwise' only applies if sexuality/interest has never come up. in this particular instance, steve rogers has so far only shown romantic leanings towards women. the argument then shifts to 'well, just because he's never said it...', which isn't necessarily untrue, but is really just a way to grasp at straws to try to make something more legitimate than it is. play him as bi. but don't expect people to agree that it's canon. same as any other fandom where one specific gender has been preferred but 'THEY NEVER SAID OTHERWISE' is the standing argument.