Martian Manhunter is a shape changer whose real form is only vaguely humanoid - it would be easy enough to give him any gender (or multiple genders) or sexual alignment.
Captain Marvel seems a bit unlikely, considering that he's actually a kid who becomes an adult when he says "Shazam" (unless they've changed canon a lot), I really doubt that DC would want to go there.
To be honest, I really don't care that much - given the number of canon superheroes in the DC universe, it would be statistically absurd for none of them to be gay - I think that the only canon example at present is Huntress, but I may be remembering it wrong.
