
While I understand where you’re coming from, I feel like you are failing to acknowledge that they most likely won’t act on the promise obsessively and if they do it’ll most likely be channeled into the smut aspect of the story. Considering the natures of ash and Lyle, I’m betting that it was more of an innocent way of them saying “I live u so much I want you all to myself”. This subtext would also most likely be hyperbolic— an exaggeration rooted in affection.

The fact that their relationship has been nothing but misunderstandings leading them here, I'm left with little option but to assume that this promise of theirs is just another gateway to more misunderstandings. Will it lead to any issues? Who knows and I hope not, but the possibilities, due to them limiting and isolating each other like this, is there. And while I never said they will act on it obsessively, I am making a statement because they are taking it serious enough that it will have an impact on their behavior toward others, in turn, towards themselves. Unless, both of them are actually lying in this chapter and neither of them actually "hate" it. But of course, that wasn't the case.

lol just imagine Ash, a once bright happy guy, after making this promise, now greets his friends/family and/or strangers with no smile or with little enthusiasm because he wanna save all that just for Lyle. Ash and Lyle are both limiting each other so they will be fine. But to outsider's perspective, it's not a good look.
Honestly what they just did is just gonna cause more misunderstandings and problems in the future. Instead of uplifting each other, they've both just chained each other down. These types of "I dont want you to etc etc at other people, only do that to me" is always portrayed as cute but in reality it's really just an isolation tactic.
Because imagine if Ash just so happens to unconsciously give that bright smile to someone he's doing business with and Lyle takes that as Ash breaking his promise and in doing so, doesn't love him anymore. Like? Problem where there shouldn't be one. Or if Ash feels like he has to constantly watch himself just so he won't break promise and cause Lyle to misunderstand him. It's just gonna be a cycle of them checking on each other and themselves for validation.