In my playthrough Ian is still playing the field, Alison and Lena are both at least roughly aware of the other, and Alison is still all about Ian.Allison is strange, the only way that she doesn't have sex with Jeremy is to pay attention to her and only her at all times, if you choose wrong she will end up in bed with Jeremy.
Even still, if Alison hooked up with Jeremy once before pursuing Ian, what of it? Ian can do exactly the same ..
One of the themes of the game seems to be the interconnectedness of people and the surprising degrees of separation underlying the relationships.