She could, but iirc it's something that she brings up herself unprompted i.e. there was no need for her to lie in the first place, plus it'd mean that she's turning down a guy she gets more pleasure with for a worse experience, which she'd be doing... why, exactly? "But she luvs him" isn't really enough of justification here imo, Alison is smart enough to realize that when the reality doesn't live up to romantic expectations and a more optimal option is readily available, then such relationship is built on a shaky foundation, at best.
Note how Lena who is potentially in similar situation (in relationship with Ian but has been fucked silly by Axel) recognizes that it may be best to break up with Ian, because he can't provide her with all she wants, and the romantic aspect only goes so far.
Maybe that's Ian's cuck instinct and inferiority complex kicking in.