Hasn't been revealed in the story but it's not a big leap to expect that MC's dad once again promised to be there for him and bailed with no explanation or gave one and it was lame, so MC got drunk to kill the pain. That's when Mia felt so bad for him that her lust overcame her sense and they did the deed.
Hmm quite interesting! Mia got any proof of that scenario so that MC can remind it? If it has been happened then MC could fetch his drunken memory and recall it. Bingo
For those who couldn't get scrappy mod "unlock all" function to work it's probably because you unpack the game using unren like me, so just transfer your saves to a new SummerHeat 0.61 game folder, paste the mod and don't use unren.