This, exactly. Jeremy plays a huge role in Ian's part of the narrative. Sure, they can have their disagreements over Alison or Ian might reprimand Jeremy for stringing Louise along, but nothing major happened that might completely ruin their friendship. Lena cheating on Ian with Jeremy would result in this outcome because Ian can't be left in the dark forever (at some point Ian's ignorance would become too much to suspense disbelief). It's one thing to destroy relationship with Wade by pursuing Cindy, at least that's something the player is prepared to deal with, but Ian losing his friend because of something Lena did, well, not only is it unfair but also creates an entirely different narrative. Perry and Wade would side with Ian and most likely stop interacting with Jeremy as well. Then it might impact Ian's friendship with Alison if she's hooking up with Jeremy and doesn't support Ian through his second heartbreak. Louise would also be ruined by Lena's betrayal because Ian would most likely want to tell Louise about all of Jeremy's shenanigans behind her back. I don't think I need to go on, it's already obvious how complicated this branch would get if it was allowed to play out.