Yeah. She is pretty sure Sakuta is besoted with her and that she won't loose.
She on the other hand fears thing might get more complicated
if Koga falls for him.
That being said I feel like TL here slipped a bit. Since
Mai: Pretending to be more than friends but less than lovers. What if you start actually liking that first year.
Sakuta: Mai-san, is the only girl for me.
Mai: I CAN'T believe that.
doesn't sound like Mai at all and goes against portrayal of this scene in both anime and in the LN. Although anime does streamline it a bit.
Especially since she clarifies like couple pages later that she fears for Koga.
I wonder if this might be a case of translator mistaking who she is reffering to and assuming she explicitly meant Sakuta while she was being deliberatelly vague.