I have to admit I used to be of the "if they're too masculine they're basically dudes," and now I'm extremely embarrassed and ashamed that I ever subscribed to that line of logic.
Asagi is that wonderful kind of bisexual, who are all cool and "equal opportunity lovers", thus lending themselves for some girl/girl action without actually offending societal norms, as such non-monogamous lifestyle usually ends in unexpected pregnancy and marriage. And girls can't "unexpectedly" make girls pregnant.
In other words, they continue to "fly from flower to flower" until something stops them. And male partners have many more things to do it, like societally approved institutions and natural child conception. Female partners can't offer them stability and social status they'll inevitably desire someday.
for what it's worth, i read the ending as torako finally caving and telling asagi what she really wants from their relationship and asagi accepting it from her
holyyyy fuck i am in such dire need of asagi/torako and this is really nice. i love it when relationships hover on the edge of friendship and romance and this has such a nice, cleansing feeling. yotsubato is my favorite manga and i LIVE for fanworks of it