Well, I believe the author has chosen this route for plot purposes but were this not be the only possible route I'd say one other route would be have the two girls love a different man. The legend only talks about how the one with their love unfulfilled would disappear but in no way does it say that the love HAS to be the same for both. So if they love a different man their loves would not go unfulfilled and they would not disappear.
Take for example two new born identical twins (no matter their gender) and separate them at birth. Take one to the country and the other to the city. After many years of living in that environment and by chance meeting, will the twins still be identical? What I'm trying to say here is that if yasuda had not taken ginka to live with him she may have not developed HER feelings towards him and instead she may have had feelings for another guy, however him being the only one that knew of the secret made it obligatory for her to be with him. Like how they say "spend some time with someone and you'll develop feelings for them". Ginka's feelings came to be from yasuda's continuous sexual harassment and the fact that she made the error of trying to get erika and yasuda together, while on the other hand yasuda didn't pay any attention in the beginning to her and instead kept chasing after erika.