Page 13 is perfection in my eyes, I don't read much manga where not even one of them is surprised or emberasssed or anything, they just want to do it and share their feelings with each other
With the progression so far and the place the characters are at, not to mention how the mc acts, i would guess that the mc would definitely not call her a slut. The argument instead, i would guess, go in a diabetes inducing wholesome turn that is if it happens in the first place.