You're 100% correct on this point and i never said it was unavoidable, i think now with Sadie coming into the picture its going to start a chain of events that leads to that very thing happening. I just don't think its all going to be sunshine and rainbows like everyone else does. The game could end with the MC not even having an option to be with Mia hence the title being "Summer with Mia" not "How i Married Mia" or "Life with Mia" or something along those lines (you get the picture).I think rekindling the sexual relationship with Mia in this game will be unavoidable because this game is named for her and this happening will be because they still have feelings for each other and the sex will either set them on a path to a proper relationship, or simply be therapeutic in helping them to get over these feelings. Either way, sex with Mia will, in my view, be an unavoidable aspect of this game.
Don't get me wrong, i enjoy SWM but i think the MC is unredeemable trash and the only reason i play the game is because Mia is great and Grace is hot.