If any eligible heroine has 100 loyalty, NTR by the king is avoidable. I forgot that not just any heroine will work. It has to be someone whose scenes are pursuable on that route. For all routes, Carla and Seria. For Stormlance, there's Leane. Hestia, you have Beatrice. Aura, it's Cynthia and Marion. Any other heroine won't work. You're given a choice between her and the king later on. This choice will be very obvious. Picking the heroine over the king will avoid NTR.
Routes are specific to what nation you start in. Stormlance route, Hestia route, and Aura route. For instance, if you want to pursue Leane, you need to have your starting nation be Stormlance. Same applies to other routes. What I'm going to say next is a bit spoilery, but concerns only game mechanics. Later on you gain another stat value called Affection. Affection is specific to heroines only. The later half of the game has no dedicated routes for any heroines. This is actually the beginning of where you pursue an eligible heroine. Again, it depends on what nation you started out in. Raising affection is how you progress scenes with a heroine. Affection also unlocks flavor dialogue for the Talk option at 10 point increments, but that's not real important. Most CGs in this game are NTR-oriented. Vanilla h-scenes do exist between Ares and each heroine, but they're few in comparison to the NTR ones.