Ayumu would seem the exception, as the redheaded figurehead of NijiGaku, but even she's got Yu, Setsuna, Shioriko, and Lanzhu.
Though if you ignore Yu's position as Harem MC, the real harem mistress of NijiGaku is Kanata; it's canon that everybody's slept with her.
(Just, y'know, literally. She uses other idols as hug pillows.)