You already got a good response to this post focused on the JeremyxIvy scenario so Im gonna tackle it in another way.
I dont agree that Ian "needs" to get one over him necessarily or that if that happens Jeremy is gonna be "taken down a notch". Depending on the choices you makes as Ian, you can make Jeremy look like somewhat of a "Loser" when it comes to women...
If Ian gets Alison for himself, the only way Jeremy is getting those big titties is if both you and Alison agree AND if you actually mention the 3some idea to Jeremy. Emma is practically locked away for Jeremy unless you get with her first AND mention to Jeremy how "kinky" and "wild" she is just to peak his interest.
Then if you throw Lenas choices, you can have him not only miss out on her but also be a detrimental factor to have Louise dump his ass
(Even if you dont factor that in, they are tied because as of chapter 11 you can fuck each others girls)
Even with all that happening, he still maintains the same demeanor so WHEN Ian gets Ivy before him I dont think hes gonna be "taken down a notch". Because hes a "you miss 100% of the chances you dont take" kind of guy.