I still don't understand exactly
why you think it's too late for it to be introduced, only
that you think so. As
dontcarewhateverno also pointed out, the setup is already there: Depending on your playthrough, Ian can already be masturbating to pictures and videos of Jeremy fucking women Ian's involved with himself. He's also subscribing to Lena's Stalkfap, and she could have a Jeremy-dildo that will very likely be involved in her content on that app. Also Jeremy and Axel have a history of taking pictures and/or photos of their sexual encounters, and at some point they will probably do the same with Lena. Which, again depending on your playthrough, are very likely to come to Ian's attention at some point. It's also striking how well Jeremy and Axel fit in with the classic stereotypical characters for what you refer to as "cuck content" – the black guy with the giant cock, and the muscular alpha ex-boyfriend who the girl can't resist going back to.
Does that mean that Ian could become a cuck later on in the game? Maybe it does, maybe it doesn't. Personally I don't think I care too much either. But I think it's guaranteed that we'll see some scenarios where there's some (wanted or unwanted) imbalance in the relationship between Ian and Lena. We can choose to have one of them romantically interested in the other, and the other not. Ian too can begin developing more dominating traits both in bed and in general, so it goes both ways – even though Lena sure seems to be able to do more without consequences at this point.
I also disagree that there's not enough "wiggle room" to develop those kind of scenarios at this point in the story. At the beginning of the game, Ian and Lena find themselves in the same situation – slowly going back to dating and having sex after painful breakups. In that particular situation they both seem unsure of what they really want, and have to gradually find that out as the game goes on. Their "general overall personality types", at least sexually and romantically, therefore haven't been "ironed out" yet because the game is exactly about them gradually ironing that out for themselves.
I don't even think it's particularly difficult to imagine how it could gradually evolve. Off the top of my head: Ian starts wanking off more regularly to Lena's Stalkfap on the nights he can't meet her in person. He gets curious as to what she shares with her followers in private, and messages her. Lena starts sending him more daring pictures, and as she's unaware that it's Ian, offers to send him sex pics/vids. Ian, at this point maybe even expecting to receive a video of him and Lena, then to his surprise instead receives a video of Lena together with Axel. Horny Ian finds it a bit weird, but the video's hot and it's not so strange that she's still got a video with her ex. And after all, he's already watched her with the dildo that looked exactly like Jeremy's cock. Horny, masturbating Ian then asks Lena when the video's from. Again to his surprise, Lena answers that it's from
last night. Ian can then choose to either go full Michael Douglas from Falling Down, or just continue what he started – the poor guy is
so close to cumming! Ian, conflicted but turned on by what he just watched, ends up asking Lena for more similar videos, until Lena at last finds out that
that guy from Stalkfap actually is Ian. Serious conversation ensues.
