Yes, the MC is a big fat pervert with an ephebophillic love for Ren. Characters in the story are also not reporting statutory rape as they should and his problematic comments have comedic uncomfortable reactions. One could argue that Ren consents and wants it, which is true, but one could also argue imprinting and that Ren never really knew anything else. I'm personally for the former, because we're taken through the story in a way you can obviously see how they fell for each other. Oh, and if anyone ever accuses any objection Ren ever had,
the reason that Ren said no those few times is because he was trying to desperately maintain a brotherly role to Haru because that's what he thought Haru wanted of him. Ren did this because he thought his love for Haru (romantic) and Haru's love for him were not the same. Which was obviously not true, but alas.
That Ren acts like an old man-child is no excuse, but it certainly is a huge reason I can read this. Ren is a very lovable character, and possibly the best kuudere I've ever seen in fiction. Haru is... well. He's trash, but I don't hate him. As much as I'd have liked him to hold out until Ren turned legal age--welp. I guess she didn't want to do the time skip. And also, as a Canadian, I'd really like to insist that people are not kissing monsters there.
If you're fine with reading a story that has that, everything else is literally amazing. The art, the characters, THE WRITING. Dear god. Like, their relationship is /profound/. How Ren so often manages to say things that just shake Haru's heart, especially. Hnngf. It's not just romance romance romance, either. There /is/ focus on sense of belonging, and relationships between the actual brothers, and between Haruka and Ren.
I was actually really glad she didn't go with the typical idealistic "oh your mother abandoned you because she had no choice" and instead went with it more realistically. Oh, and as a child that legitimately despises their mother, I can tell you right now it's 1000% possible to do so.
Basically, only read this if you're capable of separating reality and fiction (and really, young'ns of Ren's age that actually act like that just do not exist) and have no intention of letting this impact you in a way that'd lead you to harm minors. ...Unless you are a minor, in which case, I'm pretty sure depends as long as both consenting parties are of legal age? Not that you should be reading this at all, but I have no right to tell you off for it. (cough)