Just for discussion’s sake, there are girls for whom it should be clear you’re not in a monogamous relationship: Primrose on the sexfriend path; Samarra (obviously); and Evelyn and Callie, if you either set them up together or go for the threesome path. Also, I suppose it depends on everyone’s subjective reading of each character, but the only two girls I would’ve thought would be categorically against Dickon’s promiscuity are Primrose (on the lover path) and Nell—and even then, Nell looks up to Samarra so much that it’s hard to imagine she could never be swayed by her.
You do bring up a good counterargument with the Evelyn library blowjob scene, but evidently something changes with her by the end of the game, because later you can make her hook up with Callie, and even join in; Dickon’s mind meddling must be strong enough to get over whatever hangup she had about that.
While I won’t argue whether or not it’s realistic in general to hope for a good ending in this situation, I don’t think it’s unrealistic for players to assume this game is going in that direction: Belle (the pixie) encourages you to be promiscuous; Samarra is, in a way, of the same mind; and you can take actions to bring some of the girls together (Evelyn and Callie). Moreover, there are zero negative consequences to cheating on everyone until literally the end of the game; if you want to talk about realism, that one’s a bit farfetched—especially since, as you point out, you’re going out with half the village’s population. Hell, even during the ending, only the girls you don’t end up with have a problem; you still get a happy ending with the one(s) you choose.