As the topic said, I have a story in mind that may involve sex with humanoid monsters. Example, lizardmen - humanoid with lizard feature who can act like a human. Is this allowed or classified as beastility?
Humanoid creatures are perfectly acceptable. Even things like Centaurs and dragons are fine. The only way you can get into trouble is:
Real non-sentient animals
A sex scene involving a fantastic creature, which has the form of a normal animal, which could be read out of context of the rest of the story and mistaken for a normal, non-sentient creature.
( The second is an assumption based upon how I've seen the 18+ rule applied with respect to things such as age role-play in stories. )