To be fair to Jeremy, he's always given his opinion, that Lena is a party girl, and Ian will always face a lot of competition from other guys who are going to be interested in her. Although he didn't initially say she was a slut, he insinuated that she could easily become one, because of all the temptation that her career and lifestyle put in her path. And although it's possible to make Lena be faithful to Ian and avoid most of those pitfalls, Jeremy was essentially correct. In most instances, that's what Lena is like.
For Ian to be with Lena, he has to either change Lena, so that she remains faithful, or change himself, so that he can fit in, with Lena eventually becoming a slut. Obviously on the route you mention, Ian is not aware that he's effectively now a cuck, or that Lena may have started moulding him to become one openly. So unsurprisingly, that choice leads him to deny the possibility entirely, and get really angry with Jeremy. I have a really good relationship with Jeremy on all my routes, so that scene never happens, and at most Ian thinks Jeremy is wrong, but in the back of his mind, wonders if he might be right.
