feline_lump said:
It would do much the same as what the other "implied X" tags are doing now: holding all "grey area" posts and preventing conflicts over differences in interpretation, as well as providing content for people who like bisexual girls but don't want to look at penises.
But, arguably, you could tag all yuri and yaoi pictures as "implied_bisexuality", since there could be a man / girl just outside of the frame for all of them. What about the "mysterious white fluid" does make it bisexual? And even if it's cum, does it make the act itself any less yuri? There is no male participating in it actively for all we see.