The first girl Yukino still hates the MC 48 chapters in after more than 30 instances of s*x, all novels I read before this one, all it takes is 2-3 times of s*x with MC and they are all over him, this story to me, is more realistic. The slow fall is alot more realistic.
As for other girls liking him maybe too easily, it is because they are broken, and certain aspects coincide well with the MC. Nei being an example, some could argue she is using Yo-chan as an outlet for her disorder and loss of her brother, not real love, so its not like she is just throwing herself at him blindly, there is a reason for the madness.
Misuzu also I would say is using the MC to an extent, partly to get out of her own situation in life which she doesnt like (Nagisa-san hinted at this), At one point in the story when she sees the MC's penis she almost calls it her's but corrects herself. Not to say she doesnt have real feeling for him, just there are outside circumstances that made her fall for him alot faster.
Megumi may be the only person so far in the story with an honest healthy like for the MC, but we havent gotten to her backstory yet.