Sexuality is a super wide spectrum of singular, personal experiences informed by one's experience. That said, elements in Wonder Woman's backstory and the history of her attitudes to attraction in published comics point to the interpretation that Wonder Woman is bisexual. That's my argument for it.
PS: There's nothing inherently sexual to being bisexual, and I speak here from sad, personal experience. Saying Wonder Woman is bi is more about acknowledging both the character's history and the fact that bisexual people exist, read comic books, and like to see themselves represented in comic books. Considering how few characters in Big Two comics are stated to be bi, one more, and one with as big a profile as Wonder Woman, would be pretty nice.
