it's not that this Manga is about hating all men, it's about one woman who had only experienced being with horrible men in her life that made all of them "evil" in her eyes. Her father, a man she loved and trusted, brought home a woman that abuses her. The father may have known this through the prominent bruises but has chosen to ignore it and pretend everything's fine just to keep his woman. Several men he has been with to escape home had "abused" her in some kind of way. Maybe some even forced themselves on her, a minor.
So this isn't about man-hating. It's about Rio only having a series of bad experiences with men she had trusted. It had marked all men as "evil" in her books.