Well to me by sticking to much to his game and not retconning earlier parts and the happy ending he tried to provide he actually failed in the end something that was overall a great game. I will not write a long treatise about all the things that can go wrong during development of a child, how that can go wrong and get right by luck anyway since that is pretty depressing.
Just trust me that Nikki and her behavior including her promiscuity fit it to a tee, whether Sumo intended that or not. (yeah in those cases it is not a free liberated choice but a compulsive behavior to run away from things you do not wish to deal with looking for sexual partners that mostly do not suit you, well if your male people call you a player and real man and if female mostly a slut

) Thing is those things do not go away by just saying no, they are emotions and traumas that stay alive and active in the now and are not dealt with as they should have been for whatever reason. They are extremely strong shackles that need acceptance, lots of time and professional help to take care of and even then people are often only partial successful to repair the damage they did to themselves to be able to survive as a child that could not deal with what was happening at that age.
Based upon that view of Nikki, her ending was the most sad thing she could do to herself and people around her. Not because Marc would not be a great and understanding guy, but the fact that Nikki denied herself and only clunk her own shackles even tighter and that can only lead to unhappiness for her and through her those around her. Just love unfortunately does not repair that.