This is a gay fantasy, the visual equivalent of a romance novel. It sells you the fantasy of being loved. It makes you feel the feels without doing things to the characters that they don't want deep down inside. Nobody dies, nobody breaks up, nobody unloads any trauma more serious than getting groped on a train (at least until Nana tells his story), but there is the implication of things that occurred before the manga.
Nana at one point acknowledges that Kei is "not gay" (by what standard?) and I get the feeling that it's less about social norms and more about Kei's comfort zone, his internal process of rationalizing the relationship. Kei wants to get fucked in the ass, he wants to get roughed up, he wants to serve Nana. At the same time, he wants to see Nana cross-dress, and crossdressing Nana gets more pages than "boy mode" Nana. Kei gives himself to Nana rather than someone else because, according to Kei, the crossdressing made it easier for him. He's avoiding toxic masculinity while embracing a dominant partner who simply happens to be a guy with a dick. It makes you think (and it's also good fap material)
