I think you should revisit some things... to me it was more than clear at many scenes that the MC is no longer "happy" as a single "dik". He even said that himself in at least two scenes, or hinted it, maybe most clearly with Riona, cause it doesnt matte rif you choose the neutral or dik option there, both lead to a conversation about realtionship and that they both maybe should start to look for something "real".
And in the end... (except the Sarah/Mel ending) he wishes for "more" with either Lily, Nicole or even Quinn, or is pretty much "down" when you walk alone in his room to just sleep in the bed with three hot girls... realising its not that awesome.
But well, it might be only this episode like this...
To me, the MC was overall in a bad mood the whole episode. He didnt have fun at the party, only when he fucked somebody... besides the fucking he was pretty much "down" the whole time or stressed. For an episode only filled with one big party, there was not that much "funtime" for and with the MC...
And as i see it, if you dont make moves on Lily, Riona and Nicole NOW they will lose interest. Nicole and Lily are pretty much done with meaningless sex i think... the threesome was the last thing here. And in the end you had to choose. Same will go for Riona at some point, maybe already now. Sarah and Mel are just "fucktoys" and they see the MC as that too... but to me, meh... i dont care about them one bit. Finally there is Quinn... oh yeah, feelings are growing here, i dont think this will be "washed" away, it only develops or ends. For Sage... the fucking is already gone if you dont choose her in ep 8... MAYBE but its a rather unlikely maybe they will become fuckbuddys again, but i dont think so.
As i see it, the "herd" is already thinned out on the "others" path, more and harder branches will come here too. Of course the MC can stay "single" but the options for fuckable already established girls will decrease over time. Maybe thats the reason why DPC adds new girls all the time.