In Theory: Ami was bred countless times by countless Senseis for countless years in countless timelines, before the current one. Maya implied most Senseis fucked her and considering his distaste for condoms, probably bred her.
Ami may be aware of this, since memory alterations don't really seem all that effective, and she's implied she's aware of things not making sense:
And that Sensei is a "half-eaten replica":
She's also implied that she's usually just not speaking about the things that don't make sense:
And apparently just wants to be "pure":
There's also the "Ami 2" stuff:
But that seems like a whole other can of worms, related to dream babies and shapeshifters.
Overall: The BYNMG could be a representation of countless memories of past Senseis breeding Ami. Sensei also later seems to recall a memory of a past Sensei dying from, presumably, Maya Prime's handjob:
Anyway, time for some Ami Sensei Wholesomeness: