Rules are the theme of the movie. Many of the characters break rules, from the rules on the wall (against smoking in bed and climbing on the roof) to the laws of the state (abortion, licensing requirements, prohibitions on drug abuse), to rules that most people would consider fundamental principles of morality (prohibitions against dishonesty, betrayal of a friend's trust, incest, and, for many people, abortion). In some cases, viewers will think that breaking the rules was the right thing; in others they will not. Notice that there are rules that characters take seriously, like the rules that Mr. Rose explains to Homer about how to pick apples. One of those rules, is to be careful not to pick an apple bud, because then "you're picking two apples, this year's and next year's," a rule which may have a deeper meaning to Homer given his views on abortion.
And compare the way that Candy lets life make decisions for her with a "wait and see" approach to Homer, who makes decisions based on his values, including the importance of having a purpose. They have very different reasons for getting together -- he loves her, but she "just can't be alone."