Implicating bisexual → straight
Link to implication
Reason:
As weird as it seems at first glance, technically speaking in no case should bisexual be tagged without having some sort of male/female sexual interaction in the post. Additionally, all posts tagged bisexual should have either lesbian, gay, or both, but there's no way to implicate those so it will unfortunately have to be done manually. The tag itself is probably undertagged (particularly for m/f/f bisexual pairings which tend to just be tagged with lesbian or straight for whatever reason) but we might as well work with the ones we have.
Bonus: If we get these sorted out, it should be possible to roughly search for bisexual m/f/f, m/m/f, and m/m/f/f pairings (bisexual lesbian -gay, bisexual -lesbian gay, and bisexual lesbian gay respectively).
Updated