can they not learn to take a womans form or maybe some kind of magic can do that? well some monstergirls can take a human form or able to Change back in it even when they become monstergirls.
I don't think it's a matter of learning, it's more likely a genetic 'defect'. They just aren't physically capable.
Sooo...succubus minus being hellborn and sucking out the soul during sex?
That can be pretty much be said about every Mon-Girl in this Verse.
The Parasite Slime/Slime Carrier do stand out though for being the second (as far as I recall) parasitic creature after the Roper - uniquely speaking, they both have no initial humanoid female forms of their own and must infest a female host first.
The overall end result however is still 'will have sex with men, can possibly infect/transform other females into its kind' like every other Mon-Girls though.
That being said Silfs, which do you think would change the world more; Mon-girls, or pokemon, the current generation? One could claim either could be used for good or evil...I think.
There are official books by the author that go into greater detail. But they're all written in-universe and the in-universe author outright admits hes bias/pro-monster girl/an unreliable narrator (which is really the out-of-universe author's way of saying "everything is subject to change".
That said, depending on your interpretation of the books; the monster girl universe is 'grim-dark/apocalyptic' or a tentative 'stalemate' (depending on which book/profile you read but mostly leaning towards grim-dark/apocalyptic.)