Kinda like how this story doesn't fit with the whole "lesbians only" theme. As Takemiya Jin said, "this manga isn't just about lesbians. There's also bi, gay and straight characters in it." And before you say that it's out of place in a yuri magazine, she also mentions that too.
With that said, Yui seems to be the straight character.
Mayu (bi), her brother (gay), Mika (lesbian), Yui (straight), Takako (lesbian), etc.
And I really wish that panel with Mayu and Mika was a thing.