Ian can ask her which one of the two is better.
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?
Better sex isn't everything. Ian is smarter, more grounded, has better job opportunities, is far more sensible, reliable and responsible, all very attractive treats for a woman in search of a husband who she wants to spend the rest of her life with and raise a family.
Jeremy works as a barkeeper, lives at his parents and just wants to fuck around. A career driven woman who comes form a slightly higher middle class family and education like Alison doesn't respect that.
Maybe that's Ian's cuck instinct and inferiority complex kicking in.
That's possible, because the scene is fairly specific and comes out of nowhere. It isn't exclusive to limp Ian, it can even happen to an Ian who made every best choice on the Alison path.