Technically, they weren't cheating early on, because before Chapter 9 they were only hooking up. They didn't become boyfriend and girlfriend until their Chapter 9, talk in the park. So that immediately rules out Cherry and Emma, because Ian can't have sex with them and be Lena's boyfriend. Also when you're on the cuck route, it's impossible for Ian to have the threesome with Louise ( that must have been on another route). Yes, it is possible for Ian to be on the cuck route, and still cheat on Lena. But the question is more why would you? If Ian is cheating on Lena, then that version of him would never realistically be on the cuck route. Yeah you can do it, but it doesn't really make a lot of sense. Cucks tend to be unwaveringly faithful to their partners.