I would dispute several of your observations. For example, one of the things that surprised me about this game was how frequently, when offered sex early on, the MC would refuse, commenting that things were moving too quickly or that they didn't know each other well enough yet. I don't think I've ever seen that before in a porn game, and I really appreciated it. In general, the MC doesn't strike me as particularly immature; he is sincerely kind and compassionate, which is the primary thing that draws all these women in. He has moments where he is egregiously foolish, arrogant, and dismissive of the girls' abilities (see the scene where he and several girls try to move that column), but I interpreted such moments as humorous foils of the alpha male stereotype that are only semi-canonical, especially since the women are never shown meaningfully reacting to this behavior. I think there is also some character development happening within the MC; by the end of the prologue, he seems much more aware of how useless he is (by conventional standards).
That said, I do generally agree with the thrust of your criticism. Most of the women have relatively little… emotional maturity, I guess I would say. They too quickly become attached to the MC, as if they've never had a kind, interested man in their lives before. But that's just one of the inherent failings of the genre; like you noted, developing a harem solely on the basis of personal attraction is inherently unrealistic. And although I think nearly every character is unrealistically good-natured, Komisari has chosen to foster a consistently humorous, cheerful, and carefree style, and I am willing to suspend my disbelief somewhat and appreciate that world on its own terms. The only characters I truly have grievances with are Shoni, because we can't really feel the weight of the depression she clearly suffers from, and Elizabeth, whose personality is disgustingly one-sided.