It’s called “the devil’s origin”
You can read it at the link below! Ps. there’s like mpreg of sorts? If I’m not remembering it wrongly and a lot of questionable things, you’ve been warned!
https://www.novelupdates.com/series/the-devils-origin/
The main lead becomes the demon king and turns out to be the only one who can mate and bear children with any type of demon which are nearly all at the brink of extinction.
So he has sex with any really any type of demo/monster you can think of. Regardless of size etc.
It start a bit dubious but he sees it as duty.
If I understood, MC is the only demon that can be impregnated? And he has sex with just anyone, and the birth is successful, or it’s like a sperm hunt for birthing a baby?
When I mean I want to read harem endgame, I guess I meant more of established relationship where love is involved, not just fucking anyone or anything idk. Feel free to spoil me please and thank you!!

I wish there were monster harem or just men harem with mc bottoming for them, but he is the actual master. And it is all consensual, nothing like this or that one story where that guy got overthrown and raped by all of his allies or whatever.