Thanks! I would've expected Ian's possible "It turns me on" comment about Stalkfap to also be a part of it, but that seems to be the healthier version where Ian can like Lena's exhibitionism, without necessarily pushing it further than that, and they'll both know where the other stand. While "One of the best?" and the ian_cuck variable is linked to a conflicted and insecure Ian, rather than a willing and completely content cuck. That's my immediate impression, at least.
I finally had a look at the script and tried to make sense of it, and noticed that there's even some alternative dialogue for the ian_lena_sex = false variable in both these scenes with Ian, where Lena seems to be teasing him and humiliating him even more. Pretty dark. But that might just be available if they're not in a relationship—at least it used to be a point in the story where they had to have sex to progress their relationship.
AFAIK, that heavier teasing dialogue is available for both an Ian that hasn't slept with Lena (basically interchangeable with Stan) or an Ian that has slept with Lena but:
1. Subscribed to Lena's Stalkerfap (even dirtier if Lena takes special requests from patrons)
and
2. Ian has shown insecurity to get that cuck tag with "one of the best?".
I think it was also the right design decision to not to award a cuck variable for "it turns me on" for exactly the reason you stated. There are people who swing or enjoy showing their S/O off
without that masochist cuck element involving getting off at the thought of someone better in the sack banging their partner. Using that to add to a cuck variable could invariably yuck those players' yums and spoil the enjoyment of their playthroughs, which isn't great game design. It seems like the "turns me on" selection will be used as a requirement for some of that content though and will surely affect both Ian's reactions and the conflict/peace of their relationship, but won't railroad players into the cuck path. Which is a better way to handle it, imo.
For the effectiveness of that fetish, I also think it's better design kink-wise for it not to be 100% willing at first, which makes that "one of the best?" choice pretty well-designed. Unlike many games of that genre ["Hello honey I wuld like u for to be big slut now." "....Ok!"], intuitively selecting from behaviors based on both insecurity and arousal is more effective in fulfilling the kink, in which some loss of control provides a spicier element. These sort of games are best designed ideally with a little bit of proverbial mind-reading into player psychology.
Though I have a feeling Eva may go too far into some mean-spirited, professional cringe-porn direction, like she did with the awful Eric doormat route at the end of GGGB. At that point, it's not even a relationship. Just abuse. And it's implausible what would keep both characters together without love and respect for each other.
More could've been done for the variable in earlier phone incidents though. A lot of opportunities for Eva to easily do a quick retcon on invisible stats.