Chika being that is like a mixture of being too confident and less smart, so it’s likely that when (or if) she heard the rumors she just went "hehehe it was actually just this little hot moi sucking his cock the whole time". There is a chance that the thought of Sensei screwing others has probably never crossed her mind to even be denied (blissful ignorance that is). She thinks she's that good.
But I put more than half of the blame on Sensei because, he never forthright denied Chika's constant self-claim as his girlfriend like how he denied others, while actually being quite the knight in shining armor to her, inadvertently offering the false sense of hope that weights so much compared to others.
In comparison to a full throttle in denial girl, Ami really has to convince herself and Sensei about their relationship outlook so hard to a point that it feels like lying to herself. Because deep down, she probably knows how bad her odds are but has a hard time accepting them.
But this brought up something I didn't think of: the both of them only say they've known about "others", but how much exactly does Futaba know? Makoto knows about Miku first hand, but who does Futaba knows? (unless this is the wall too thin to hide everything situation or an extremely logical conjecture based on Sensei's demeanor)