It's Aglaecwif that said the ones from the Body lineage are perfect for breeding, for obvious reasons. Also I think that considering how many monsters and how many are specifically made for breeding purposes, we can easily concur that a lot of superhumans throughout history have fucked them. We also gotta point out that Xanthe said that the only hope for breeding for a superhuman is found in monsters, I'd personally say that that statement means some wild experiments went down
How many monsters are really meant specifically for breeding though? I can think of exactly one off the top of my head that we've seen so far (though I understand why, considering the game is very combat-focused), and even that one is kind of a stretch, being the monster that infected Angelina. And again, it really wasn't focused on breeding.
Monsters are supposedly the only chance superhumans have if they wanna have children, but that is most probably not true for all of them. I vaguely remember that superhumans can't have children because each one is practically a different species, meaning they are genetically incompatible, even with someone with the same monster parent.
So what makes monsters more compatible for breeding than superhumans? If anything, the genetic compatibility should be even lower. That implies only certain monsters, with traits and abilities permitting, can cross-breed. Either that, or monsters are literally built for cross-breeding, which wouldn't surprise me, considering how many species there are, and with how many variations of lineages. At the very least, they're probably more suited for it than superhumans.
Regardless. Finding a monster you are both capable of, and willing to, breed with is probably much harder than it seems, what with MC, a literal hentai harem protagonist, with literal hentai harem protagonist genetis, being our point of view. Most of the monsters are not exactly conventionally beautiful, and their fertility is questionable. And even if you found one that is both capable of breeding with you and not too ugly to stand, it most probably is not docile towards you, so you either have to beat it to submission and commit bestiality...monsteriality?, or lose and hope it both fucks you and doesn't kill you afterward.
And even if you do survive the process, the resulting child is by all means a monster, not a superhuman, so I assume that reduces the number of applicants by a lot.
I don't doubt many... experiments... were conducted about this subject throughout history, but it's probably lower than you'd expect.
