Why the fuck mangas do this all the time.
One that's really bad is the guy who's an 'orc'
But he's just a human with orc blood and no orc like features.
Slime reincarnation. Just a anime fuckboy
Spider isekai, Generic anime girl
Zombie princess. Just a girl with no zombie features.
Happens in manwha to.
Give me a good manga where they stay monsters.
I feel like Shiroari earned it (or I did feel like that, then I read the ending).
Anyways, here's some that would qualify to various degrees as "monsterfuckers". Hentai is excluded because that actually is common enough in that genre.
Implied Romance:
Skeleton Knight in Another World - idk if he gets his human form in the LN, but currently he's skeletal
Skeleton Soldier Couldn't Protect the Dungeon - I think the author's on crack so there's no real plan as to whether he humanizes or not, but last time I read it there was some seriously implied chemistry in one loop
The Gym Teacher that Dies at the Beginning of a Horror Movie - gorilla man remains gorilla man. It counts. Also I'm pretty sure the cashier is fucking the Dragon Ball alien.
Those With Wings - Short story with living fuzz ball as protagonist
An Extremely Attractive Gorilla - Kinda like Cat Fucker but people aren't as vocal about it so it stays in this section
Slave-chan and Tentacle-san - the tentacle is a girl but you can't tell because it's a tentacle
Yuusha wa Shimei wo Wasureteru - Demon Lord is never shown as a human
The Mutant Wants to Kiss His Human Girlfriend - mutant is very mutanty
Bride of the Non-Human - Somehow extremely gay despite the monster appearing genderless
Off-Screen/Side Characters
Kemono Michi - the guy's a zoophile and has already sexually harrassed several monsters and monster people. if he hasn't stuck his dick in mythical beast I'll be very surprised
Love After World Domination - one of the side human villainesses is in a relationship with a bear monster, who I believe never returns to human form
Lazy Dungeon Master - most of the non-humans have humanoid forms but not Ittetsu the salamander, who has a dragon (often in human form) wife
The "They're Doing It For Real" Section
I Am Behemoth Of The S Rank Monster, But I Am Mistaken As A Cat And I Live As A Pet Of Elf Girl (aka Cat Fucker Isekai) - As far as I know, big monster form and small cat form (and it looks like everyone wants to fuck the cat, as implied by the clever shortened title)
A Friendly Orc's Daily Life - The elf sexually assaults him
The "So Goddamn Close to Perfection" Section
Minotaurus Sweetheart - Despite being a small web manga with a mid story, this was the hopes and dreams of monster fuckers everywhere. Every chapter had people spamming that image of a hamster trying to eat a banana. And it came close. So goddamn close. If only it ended 3 pages earlier.
Anyways, I know you were going more non-human isekai protags, but I kinda got distracted and went the monster fuckers route, which has some overlap.