They most certainly do drop from field battles (and siege defenses, I think). And yes, it is easy once you realize having sex with non-princesses raises reign affinity. Sell a progeny to the brothel if you're impatient.
Which doesn't work half the time, and is pure RNG if sexable NPCs even spawn or not.
For example, this last run I did, in 6 gameplay hours not a single fuckable goblin spawned. I still can't even interact with goblin princess because my reign affinity is only 1 (Lizz is never up to fuck either).
So if I picked a Monster Eater knight, I would have never climbed out of the negtive affinity. It's pure RNG.
Treating everyone drinks in Faun's tavern is the best (but still purely random) way of getting reign affinity. Doesn't help that most of the time tavern patrons are dipshits like Lamias or Driders who don't have their own reigns.
Now you'll suggest another game-breaking exploit, or to use cheats - but then why even play the game, might as well just google the fucking CG-rip.