Pope Paul VI, in his Humanae Vitae (1968)said that, "It is always intrinsically wrong to use contraception to prevent new human beings from coming into existence." (source)
The words of the Pope are explicit. He said that contraception (= preventing new human beings from coming into existence)was always intrinsically wrong. In other words, every form of contraception must have always been intrinsically wrong per the Pope's very own words.
How do Catholics explain the stance of Pope Paul VI on contraception?
NOTE
This is not about the natural or artificial kinds of contraception. This question focuses on the encyclical of the Pope entitled "Human Life" (1968).