This is an old Australian joke. It might help to understand that a century ago, the methodist church was one of the parts of the temperance/wowser movement.

A methodist priest comes home and sees his son reading the bible. "Daddy daddy", he asks, "I've been reading the bible but it's really hard. Why is contraception a sin?". His father replies: "Well now that you ask my son. Using contraception might lead to sex, and sex might lead to dancing."