You probably don't have enough of something. Maybe living space for newborn monsters. Or resources - each ordinary monster except goblin needs a resource specific to that monster, while heroes all need their own resource to remain fertile after mating.
Problem is you need good Wifi to get the translation... i have shit wifi so translation takes FOREVER and not worth it for me
Apparently, no. He can have sex for pleasure (and apparently, he does it a lot), but to get monsters for fighting, he has to breed prisoners with monsters.
The Naked is a Research you unlock. The rest are random loot after raids. They have very low chance so be prepared to not get anything.