Most official online dictionaries are corrupted by "gender identity". In the not too distant past, gender was not associated with identity. You either were either male or female and that was your gender(I didn't matter what you identified as), which to most people, was synonymous with sex. And it still is to some extent.

Urban Dictionary is surprisingly more useful for questioning bisexuals than these sites, because most of their definitions still express the binary definition of bi, and if they use the word gender don't portray gender as an adopted identity but just another word for sex.