In my headcanon, Diana is absolutely bi, no question.
I had written myself into a bit of a corner about it, however, to be honest. In my interpretation of Diana’s origin, it was very specifically stated that the amazons of Theymiscira ALL thought of Diana as their daughter, not just Hippolyta’s. So that meant even flirting with another Amazon from that island would be awkward. And most of my stories took place there.
But I think she’s bi, and it’s been hinted at in canon for a long time. I hinted at it a bit too, and we did ALMOST get to marry Hippolyta to Philipus. It still kills me we didn’t get to do that, DC was willing.
But my run ended before we could make it happen. It would have been the best scene ever. TEARS were guaranteed. Dang, now I’m bummed!