Actually it originally refers to having two sexes. Aka hermaphroditic.
I really hate how the word “sexual” has evolved in meaning into “relating to the activity that evolved to happen between the two main sexes”. We have the perfectly good Greek root “ero” and for some reason we have gradually muddled up the meaning of “sexual”.
It could also be applied to animals meaning something that nowadays would be described as “intersex”.
I just think this meaning makes much more sense than “being attracted to two (later multiple) sexes”. If about sexuality and not sex, “bisexual” should just mean “with sexuality related to the number two” based on the etymology.