This doesn't get too deep, and the protector/lover, atonement/genuine love angles weren't explored that well, either. It gives the impression that it was written backwards and that selfishness is being passed off as romance: Yukihisa and Kengo get to know each other after they've already been living and sleeping together for awhile, and Kengo pushes Yukihisa into that in order to alleviate his own guilt. Neither of them is developed much: Yukihisa is one of those one-dimensional woobies that you can only feel sorry for, and Kengo broods and angsts while not being too interested in Yukihisa as a person, rather what he represents (doesn't know his plans for the future, his past, his likes, his grades etc.)—and when he realizes that, it comes across as dumb, because he's 29 years old but written like an ignorant teenager who throws a tantrum when Yukihisa tells him that he wants to decide his own future.
There's some poorly-written incest-rape for shock value, to make Yukihisa even more pathetic and pitiable, and to ensure that Kengo swoops in to save him. The same Kengo who has sex with Yukihisa for awhile, yet only starts to consider his pleasure after he finds out that Yukihisa is a rape victim...while admitting that Yukihisa had "looked like he was hurting" all along. So I guess our 29yo protagonist was OK with having sex with an 18yo who wasn't really enjoying it, and was only concerned with himself until he felt like the rapist's (cartoonish, tbh) gloating hit a little too close to home?
But, then again, this is the second Nakamura Shuuko work I've read where she uses rape and dubcon sex like this, so I'm not surprised. It's yet another instance of rape used superficially, for quick drama, that was a popular go-to in the 2000s. IMO, it's dated and deserves to be left in the past.