A mix of things I think.
Assumption that Nozomu was a teacher like Sekai and Sensei (don't recall that being said), that he cheated on Sekai with Sara, was Sara's boyfriend/teacher, and the fact that HOPE seemed to be implied to be the first god which a Narrator called Nozomu, and it seems like HOPE and Yasu are fucking with Sana in particular. There's also how a HOPE-like creature could be interpretated as having fucked Sana's "whore mother" as it called her, since it claimed to have tasted her fluids, etc.
I think people also conflated "Papa" from the UDH stuff with Nozomu, and there's some "do what Papa says" stuff, that sounds cultish.
Overall: It's a reasonable theory, but there's not really anything that supports it yet, as far as I can tell or remember. Not to say, that the likes of Rin, Sana, and Nodoka won't turn out to be related to Sensei.