Granted that the depiction of bisexuality in the canonical DS9 MU episodes was somewhat heterosexist -- essentially, women are bi to titillate the heterosexual male audience members.
But, one thing to consider about Kira, Leeta, Ezri, etc. -- these women are all non-Humans. Sure, Humans have the trait of being consistently and strongly attracted to a particular sex(es), which we call sexual orientation. But who's to say that the concept of distinct sexual orientations applies to a Trill or a Bajoran? Maybe all Bajorans who have gone through puberty have a strong and consistent sexual attraction to both sexes, and are therefore what Humans would consider bisexual. Or maybe the same is true of Trill.
After all, there's still controversy today over whether sexual orientation describes something biological and innate (the essentialist school of thought) or something that exists as a cultural construct (the constructionist school) -- and mind you, these schools of thought both exist within the LGBT rights movement; not all pro-LGBT people conceptualized sexual orientation as something innate and biological. So why would we assume Bajoran or Trill sexuality would be any less complex -- let along Mirror Universe Bajoran or Trill sexuality?
That's not to say that the canonical DS9 MU depiction of same-sex relationships was itself particularly great. But in the abstract, there's nothing that necessarily requires us to think that an MU character (especially an MU non-Human) who has a same-sex relationship must be of a different sexual orientation from their Prime Universe counterparts.