I come up with some new tags:

• bi/male
• bi/female

Explanation: bi refer to "a male and a female" (binary gender), and /male refer to "engaging with a male". So bi/male means the middle one is a male and he is engaging with both male and female by his side.

That may not be how bisexual worked, I know, just whatever. If the above suggestion is practical, then here are more tag combinations that can be considered:

come from pansexual, can refer to "a binary gender and a intersex" or "a intersex and another intersex". (There are alternate words such as omnisexual or polysexual or whatever.) I do not intend to use sexual orientation vocabulary as the tags, I just didn't find better words.

ok I don't know how do I need these tags, so I just dump this idea here before I forget.

I think bisexual is currently kind of a weird tag, given that we tag non-binary sexes here.

Currently it has ~2700 taggings. Probably it's quite undertagged, but I think we need to give that small number some weight and ask: 'if we split this up more, will it result in better or worse tagging?'

(My answer would be 'probably worse, since it increases the complexity of the process'.
Personally, I'd prefer to just replace bisexual with something more general like polysexual)

In any case, there is also one other question that's relevant here:
"What do you actually want to search?"
(ie. what distinctions do people actually care about in a practical sense?)

I get what you're trying to do here, but it's confusing. You're using bi to mean two characters of different sex, but the word doesn't connote that anywhere else.