What is the concept behind 人不坏，没后代? I've come across a few variations of the phrase:


人不坏，没后代


including:


猪不坏，没猪卖，人不坏，没后代！
男不坏女不爱，人不坏没后代
人不坏没后代鸡不坏没蛋卖


Perhaps it is the act of copulating itself that is considered bad? I'm not totally sure. What is the concept behind 人不坏，没后代?
I asked a few of my students about this, and they mentioned something about sex being bad, too.

In a nutshell, there's nothing that doesn't have some bad aspect to it, even the best things.

One student specifically mentioned the second variation, 男人不坏，女人不爱，没有后代: "if there weren't bad boys in the world, women wouldn't fall in love, and that would be the end of it, wouldn't it?".

As for the variations about the animals, maybe it's just for the rhyme. But the one about bad men/boys, you seem to have the right idea.