So, from what I'm gathering, forgive me if I'm wrong, but it seems like the mother is the main source of Mayu's fear of men. Not sure what's happened to her in the past, but from the apparent lack of a father, she seems to be pushing her hate on to her own daughter.
I certainly hope mom doesn't become a primary antagonist when Yuki is revealed as Male. Along the lines of Yuki "lying to Mayu to get close".