I get what you're saying, but checking for the status in other relationships is a pain in the ass from the tracking perspectives. I agree, that Ian jacking off to pics of Jeremy doing either Alison or Emma could have an impact on this route in the future, but locking trigger conditions behind those actions is not wise, because there might be players interested in roleplaying as a cuck without ever showing a sexual interest in Alison. So, Eva introduced a pretty self-contained opportunity for Ian to be curious about Lena's sexual adventures / submissive in bed.
Alternatively, some could argue that jerking off to Alison and Jeremy's pics doesn't count since it's basically free porn starring his friend (which Ian can have no feelings for) but dating Lena on the other hand is very different, since it's more serious and entails certain expectations. For these type of roleplayers it would suck to be railroaded into cuckoldry with their main girl just because they were open to seeing their bro do it with a totally different girl.