Just throwing out an idea, but to make the choice even more noteworthy. Perhaps choosing to go with a town NPC would prohibit the player's character from getting with any monster girls or the towns other NPCs. Allowing the player to possibly trade the bonuses of possibly a dozen different stat boosts and modifications from completing the quests of many monster girls, for a massive game changing perk in the end game that they'd get for going with the important town NPC. In the context of the story helping a girl in town would require more time, effort, focus from the player character than just doing whatever challenges a monster girl feels like throwing the character through. Because they'd want more of the Players attention since the Player's based in town and their requests would span the entire game instead of just the local area of the monster girl.
For example wooing the guild girl would somehow allow you to pay off your debt more easily, going with the wizard girl would give access to possibly exotic spells, helping the priest girl with the church would give possibly more divine bonuses.
