We call them monsters girls, but the most important trait is they are still human. We identify them as human, before define their particularity of a monster.
In fact it’s like cosplayers or people who changes there body on purpose or not, in that case, but at an extreme level.
Example with a cosplay of Kerrigan:
In a near future with the evolution of surgery and cybernetic technologies, it would not be surprising to see people changing their members, their skin, bones structure and skull covering, for looking like one of these creatures. All of that doesn’t remove their human belonging.
Then, a lot of people are attracted by the exoticism and the originality.
Nothing wrong with that, it’s not just a preference (it can be), but it’s also an instinct. Our specie evolved by mixing our genetic material. Keeping useful genes and lost dangerous one. For example we carry about 8% of virus’s DNA in our genome
Endogenous retrovirus - Wikipedia (that’s why a pregnant woman doesn’t share toxins in her blood to the foetus, for example). It doesn’t mean we are attracted to viruses, but naturally there is a selection and our brain will tend to identify a human looking different to us and more the difference is highlighted more we have an attraction or interest to this one.
Another example, when we share saliva by kissing, Swapping spit and testing chemistry: How kissing, germs help you pick your partner - National | Globalnews.ca brain is able to analysis the microbian’s flora of our partner, and detected if we share the same. We feel more good feeling when the microbian’s flora is different than our own.
Adding to that it can be something else than a genetic mixing purpose, but we can enjoy to be with a person able to protect us. What is better than a monster to protect us against any other threats? Thanks to her super powers or physical abilities! The need to feel protected isn’t an exclusive female’s trait.
Also, there is the character, a monster girl, can have a personality fitting with her original appearance.
A cat girl would purr when happy. Something cute like that:
Or at the opposite a vampire will have maybe a predator behavior. That’s fantastic and maybe fun!
In conclusion, if you love or not monster girls, there are nothing wrong with you, we need both types of people for keeps our specie going!