This really bothers me.
That some women are only introduced into the series for the sake of being villains. Those make some pretty bad episodes, but interestingly enough, the people on Reddit have seen male villains and actually want them to have episodes centered around them like Sheldon who tried to murder Bob by hitting him with a BB Gun while Bob was dangling of the side of a roof!
Meanwhile, here's a list of the females introduced just for the sake of having a villain in the episodes.
1. The horse riding instructor
2. Fanny
3. Sally and Trish
4. Cynthia
5. Lunch lady
6. Jackie
7. Tammy
Let me know if I missed anyone.