Agree with this wholeheartedly. From a character standpoint, there's no reason why the MC should be refusing Cathy without the player choosing to do so just because he's chosen to be with M&J. He can be fucking Sage, trying to fuck Jill and Bella, and have fucked Camila, Mona, Riona, Quinn, Nicole, and Sarah at this point, 2 of which he could have fucked at the Prep party just hours earlier, and there's no sign he's going to stop doing anything with them just because he's with M&J, so this makes no sense from a character and narrative standpoint.
The only reason most people have been able to prescribe to it is that Cathy and Jade are basically substitutes for M&J action on the DIK path, but even that seems odd because it assumes everybody on the DIK path would want that yet players can choose to reject them as easily as they can reject pretty much everybody else. So if people on the DIK path wanted M&J but didn't want Cathy and/or Jade, then it's not much of a compensation if they already rejected them.