While this is not usually my sort of thing, bit extreme for me, I crafted this little plot for someone else, and then they decided not to use it, and I thought I might want to give it a shot.
In the kingdom there were three princesses. The eldest was considered very well behaved and the very essence of polite and regal. She had a very nice, arranged marriage and was considered by all to be gorgeous and pure. The middle princess was viewed... Less pleasantly. While she had a good public face, there were rumors of dalliances with serving men and even other women's husbands. She was still graceful, and none were certain whether they should confront her on her deeds. This had had a marked effect on the youngest princess who had actually seen some of her sister's dalliances first hand, which had made her quite curious about sex and men, but she did not have the skill in seduction of her sister, and her father was careful about guarding her. Still, it was deemed that she was too negatively influenced by her sister, and she was sent off to be raised in a nunnery.
On the way there, though, she managed to corner one of the night's watches, a young man, uncertain of how to handle the princess ordering him to disrobe, uncertain of how to rebuff her advances, and she finally manages to obtain the pleasure she had seen her sister take. This proved to be quite unwise, though, for the event attracted the attention of a demon, who captured her while the guard lay, discombobulated from his climax. The demon stole her away to his castle, where he planned to use her royal blood and budding libido to breed and create new demons, new horrors, all while enhancing her sexual lust to ensure that even if she tried to escape, she was unlikely to get far.
How played up the pregnancy/lactation/whatever is is very much up to my partner in this game. I was hoping to start it with her emerging from her wagon or tent where she was resting for the evening in camp and cornering the young watchman, and go from there. I'd be glad to tweak the idea, add to or manipulate the details depending on what other people desire.