Jessy would be the closest to a monogamous play through, but I'd bet there's some stray somewhere along the way that you can't avoid currently. I'll make even Jessy optional when I remaster the game. I'm going to make the game as flexible as possible for the player.
Admittedly, as a lifelong, unapologetic man whore, I've written the story from a man whore's POV. Since it was written with harem in mind, it may feel pretty hollow if you only focus on one girl.
It won't be difficult to not bring Jessy for the ride or make Jessy a true wing person rather than an LI. That was kind of the direction a had planned when I was going to do a much darker path.
Even as big an asshole as I am, going really dark just isn't in me. I think I can do a true romance path with small number of
intended LI's that doesn't go dark. (But not every side piece or random hookup in the game)