As it currently stands, Lena won't commit to a relationship with Ian if she's fucked Jeremy on the first opportunity available ( when louise goes out and leaves him in her room), you do get other chances later, if I'm not mistaken.
At that point in the game Lena's just not that messed up in the head, it happens slowly as you commit to the route.
The cuck route is pretty simple, reject sex with Ian but still commit to a relationship with him, sleep around for the entire game, and as Ian look at Gillian's pictures at the start of the game and make a pretty obvious choice when he dreams of her later on, "move on" on ch12. When Lena's giving handjobs and such ask about her experiences with other people. Strictly speaking you actually can have sex with Ian and start rejecting him later and still be on the route (just not the "pure cuck" route), I think the trigger is actually Ian asking for her experiences and a choice after meeting Gillian.
All in all it's one of the less enjoyable routes, I've found, to me it's pretty boring just going through the motions as Ian on it. Beats Robert's though.
Pure lovey-dovey, rainbow-puking, cutesy Trinity route its the true(tm) canon path.