Thinking is hard. OK, I'll try to help.

Somebody said, "The reason young people are leaving the Catholic Church is they don't like its doctrines on sexual morality."

The contrapositive of this is, "If young people didn't dislike the Catholic Church's doctrines on sexual morality, they wouldn't be leaving."

A counterexample is, "Here are some churches that have different doctrines on sexual morality, but they are losing young people like there's no tomorrow."

Therefore, it's not the doctrines on sexual morality. Young people are leaving their churches for other reasons. 

Think about it. It's really quite simple.