You must start off with a female Sim. Preferably young adult, but any child-bearing age is fine.
Your sim must have the Evil trait. It will also help if she has the Master of Seduction Lifetime Reward.
Here's the story: Your Sim hates men. Like, really hates men. And now that she's in charge of the city (she doesn't have to be in any kind of political position like Mayor, but it would add a little authenticity to the game) she wants to wipe out the entire male species.
Male sims can be used to produce heirs, but once they've been Woo-Hooed, they are useless to your Sim and must be destroyed. When encountering a male sim, you have two options. Try flipping a coin!:
1) Heads: Use testingcheatsenabled and Create-A-Sim mode to turn them into a woman.
2) Tails: Kill the sim. Literally kill them.

If your sim gives birth to a female baby, that baby becomes an heir. If she gives birth to a male sim, he is put up for adoption. Basically, no boys allowed.
The challenge is completed when every sim you encounter is female. No male butlers, repairman, townies, etc. Everyone should be female.

POINTS
+10 points for every daughter your Sim has
+10 points for every male Sim you kill
+50 points for every male Sim you successfully kill WITHOUT using cheats
+10 points for each grave in your inventory
-50 points for every male Sim who naturally dies of old age. If a male sim dies of old age, he has successfully lived a full life and that will not do.

Happy Simming

I think this challenge could actually be really cool! I think I might try it out in a minute.