Relying on the dense protagonist is never a good move. The problem is that your not good enough to write clever things so your protag has to be extra dense to make up for it.
Prediction: the "slut" isn't a virgin (whatever), but I'm also guessing the seiyuu thot that is trying to fuck with him is an actual prostitute. It's just to cliched.
Honestly I agree with the younger sister that the mc sucks ass but I don't agree with her because I the female lead sucks too. If you're wondering why I'm still reading this I just read things I hate a lot.