I think the bisexual tag should only be used if there's bisexual romance (two girls and one boy/two boys and one girl, for example), not for bisexual characters.
That's Polyamory
or Threesome
, not Bisexual
.
To be honest, you're both right...
That said, after some reflection and going through the comments section in depth, I've begun to think that there is actually a deeper message in this story.
Honestly, it's seems like the main message is about how people can sometimes end up deceiving themselves. The interesting thing is that the "truth" of Mitsuko's orientation is ambiguous/left up to interpretation... which I can understand why that would be upsetting to a lot the readers. But, I think the point was more about the fact that Mitsuko didn't know who or what she wanted to be, which lead the ending in the story happening.
Of course, there's also the issue of how the other women in the office are completely duplicitous in what they're thinking versus what they're saying. Really, there's a part of me that's inclined to think that the "real victim" in this story was Kyouko. I mean, people talk about her behind her back, and in the last panel- based on her reaction- it seems that she actually cared about Mitsuko a lot more than she realized.
Hence, you could say that she's the victim by not being honest with herself and Mitsuko when it mattered most.