I mean, most birds don't lay unfertilised eggs outside of mating season, that's mostly a thing with domesticated birds, since it's a tremendous waste of energy. I guess harpies could be self-fertilising, since they're a female-only species, or do they reproduce parasitically? Same goes for cows and milk, only pregnant cows start lactating (though their bodies can be made to believe they're pregnant artificially via hormones and they can be milked for months after giving birth), also pretty weird that the muscles just vanished from the guy while the other two female minotaurs were pretty buff, but okay, different breed, I guess? Finally, for most species of fish, the female ones are bigger and stronger, including most that can change their gender (clownfish, for example, can change their gender after all being born male or of a neutral gender, but the one that turns female is the largest one and the dominant party).
TLDR: monster girls and biology don't mix well, screw it, it's all magic.