Here's a list of 18 such cartoons where 'sex' made a guest appearance: 1.
There are many shows that subliminally added sexual content, much to the horror and dismay of the parents who were smart enough to spot it. Or maybe the movie makers decided to have an inside joke and threw in some sexual imagery just for a few laughs.
In hindsight, I think cartoons aren't made entirely for the younger generation, they are meant for the grown ups as well.