This damn MC is so pathetically useless that he's ruining this manga. Why do the Japanese want to read about the non-love life of losers?
Do you even understand the premise of this manga? It will end if she bangs him.
Also, MC is fine. Mia introduced herself as perverted maid and she appears as she has a lot of experience and had a lot of partners (we now know she is lying but MC doesn't know that for sure) - you wouldn't want to bang a person that leaves a first impression like this. Imagine a perverted stranger spreading legs before you, harassing you into sex, stalking you... Most people would get disgusted or worse and would want nothing to do with a person like this.
So declining her is nothing strange. Sure, it's mostly for the sake of "romantic comedy" and it's kinda like a competition now (he'll lose his pride if she bangs him) but still.
Mia is perverted in a cute way in manga - to us, readers. To MC she appears just like a real-life person like her would appear to us. Not a relationship material, probably should even avoid one-night stand.