Very impressive game -- great artstyle, and the dialogue is top notch in this category. Kudos to the author Eva Kiss!
Which is why it may be paradoxical that I want to point out something that looks like a (small?) flaw to me. The writing is so good, when something doesn't go perfectly it becomes noticeable.
If you enter the cuck route with Ian and Lena from a more typical love story route (say, via the "one of the biggest?" [cocks] option when Ian and Lena are having a dialogue after sex), Lena appears, all of a sudden, to change her sexual behavior to Ian. When before she was lustfully whispering to herself how perfect Ian's body and especially his cock was, now suddenly she's all into denying him and teasing him. I understand that this is probably meant as her doing what she thinks he wants (and apparently he really does in this route), but this makes her a bit ... mechanical. Can she change her desires that easily and that quickly?
My impression was that that Lena was probably designed for a different path -- say, one in which Lena and Ian didn't have sex (or had it only once and very late), so Lena didn't wax poetically about how great sex with Ian (and especially Ian's cock) was. So it wouldn't feel like she was denying herself something, especially if, in this route, she was getting sex from other guys. But to just default to this Lena, even in the case in which she and Ian had lots of sex before suddenly veering into the cuck route, feels a little... jarring.
Perhaps specific dialogue for this set of choices could be written instead? Something like a frank conversation between them about this (and they're such a nice couple to each other -- how come they're not communicating about their sexual preferences? Even after Ian said 'they could do some experimenting together'? I had thought that choice would lead to Lena and Ian opening up to each other about their sexual experiences and preferences, but nothing of the kind happened). In such a conversation Lena could give more information about how and why she's changing her behavior. Or maybe she could have a monologue in one of her alone viewpoint scenes, talking to herself about how things are going with Ian and explaining better what is on her mind and why she is giving up on his cock.
To me, feels unfair to Lena, more so than to Ian, who may be getting what he wants. Still... even to Ian this seems a bit strange. He was all into having intercourse with her, and how great he felt inside of her, and now suddenly he kinda likes being denied? Some explanation is necessary here, too. (It feels to me as it would if we suddenly found out Jeremy was dating Minerva... it's not impossible, but one feels the need to know how and why this happened.)