It wouldn't be surprising if Batman (Bruce Wayne version) was Bat-curious. I'd believe he is bi but not exclusively gay: he's hooked up with too many women (like, almost all of them villain or not), but he's pushing 40 and living with a college-aged guy, so I'm going with Bat-curious at least.
Wonder Woman's always been kinda bi in her various incarnations (comics and her TV show had hints). She was always about Steve and also had a run with Bats, but has always had a soft spot for some of the ladies from her home island. If memory serves, weren't the original writers/creators of Wonder Woman a bi couple? (too lazy to look it up, sorry).
My guess it'll be Firestorm, Hawk (repression-galore), Plastic Man, or Martian Manhunter.
On a related topic, Firestorm's face is on fire and he doesn't even care!!!