None of them really come out looking great. Sachiko had the worst reason for marrying Amano, and the marriage became sexless after Miyu was born, so it was messed up from at least that point, if not at the very beginning. And neither of them did anything in the 20 years to fix it, so the blame goes to both parties, not just to any one of them. In my personal opinion though, I think Sachiko comes out looking a little worse, because of how this all started. I remember her little self-satisfied smirk after half-forcing awkward and unsure Amano to say he loves her. The marriage was only a short term bounce back solution from her terrible relationship prior, and the dissatisfaction ate away at her quickly enough to withhold sex after only a couple years of marriage. But marriage is a partnership, and Amano should have done more to fix it, except he was a non-confrontational wimp with confidence issues, so it was easier for him to just ignore it. Really, it's a text book example of how not to get into a relationship and how not to run a marriage.