My opinion

I've read too many yuri mangas so this what i think will happen.. ╮(￣▽￣)╭
: Touko actually like mayumi since they were highschool(just becuz) and whe she heard that mayumi got a fiancé she got mad and seduced her fiance so she would break up with him, maybe she also lied about the pregnancy so she can breakup with her fiancé after they broke up..... Idk man I'm making my own story here HAHAHA ε=ε=(ノ≧∇≦)ノ