I always thought of everyone on Pandora as bi - There's the Wildlife preserve with two test subjects and a researcher who were at least gay, Hammerlock and the dude that got eaten, the odd character in Sanctuary, pretty sure that woman from overlook is and I bet a few more. I dunno, I honestly don't really pay attention to it, like I said, I just assumed everyone was bi, just the kinda vibe I get from it.
Also, I like to think human sexual drives, and encountering new species, would lead to pansexuality in any sci-fi setting (a la Jack Harkness) , just seems natural.
Anyway, as to which Vault Hunter? Pretending I didn't already think they were all bi, and taking any hetero relationship to bar them from coming out as gay, then I'd say Brick and Zero are the obvious candidates -- and Axton HAS to be bi, I don't think that's me, I think most people probably think that.
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