I understand, more or less.
You can complete the game by staying completely loyal to a single girl except, actually, the mom. In the full game you will only be able to unlock the mom's route AFTER you complete the 3 main routes, that means Charlotte, Julia and Valerie, likewise Eileen's story will be unlocked once you complete all the previous ones plus the mom's story, but each time you end an story, you are supposed to start over, since each story with each girl is it's own. The game is supposed to have you focus on a single girl, but the original investigation, the first 3 days with each girl, are there so you can delve into their stories and make a final decision of whoever you want to keep going with in the forth day forward, as in Charlotte's case.
Furthermore, the first scene will never be sex, but most likely teasing: You tease Charlotte in the bathroom, Julia gives you a footjob, you masturbate alongside Valerie, and in mom's case, she gives you a handjob, and only in Julia's and Mom's routes you get to have sex in the second day, but only inside their psyche. This is done exactly for the reason you are mentioning, I don't like cheating myself, I see why you would think it would suck, but if you want to avoid it, you can do this: Make 1 save file per girl, that way you can investigate all of them without cheating anyone in a single save file. "Cheating" on them only slightly changes some dialogues in their routes.
Lastly, I will implement cheat codes so you can bypass stories if you want to. I don't really advice this because the last story, Eileen's story, heavily depends on all stories combined. I hope that this clarifies your doubts about cheating on them, mate, I try to make it so there isn't any cheating at all. If you have any suggestions about it, go ahead.