This has told me all I need to know. I know there will be people that love your game and from experience, I know the writing process to be a good and telling your story is important and I stand by that. All that said, there isn't a single thing about a cheating girlfriend that I like.
I don't like the thrill of "is it true, is it not true?" nor do I like that you are confirming that the Ex/antagonist already has the upper hand on the MC without any effort (dickwise) nor do I trust that you still want Harem game fans to still try your game instead of letting them leave.
You said this is not a typical En Tee Argh story, but just few points above show that it's pretty typical. MC cock small, not only that, but smaller than the antagonist, the girlfriend is cheating (maybe not, you haven't decided yet) and all this happens no matter the user input. It sounds pretty typical to me.
To be more fair to you, I appreciate that you have all the tags in order and that you are taking questions seriously now (saw your first two posts :/).
If you wanted it to be atypical for the genre, make the MC's dick wayyyyy bigger than the other guy and the guy is just a smoother dude than MC. If you want it a little more original, make the girlfriend cheating story involve MC. Maybe MC was the one cheating and the girlfriend getting revenge. Shit like that. If you set it up the same way, there is no way someone can give a different perspective on first look. That's just my advice.
Anyway, I wish you all luck and hope that you gain the success you need