How do Catholics explain the stance of Pope Paul VI on contraception? 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).
Natural contraception leaves open the chance that life may begin due to some unforeseen occurrence like out of monthly cycle an egg was released that could be at the right place at the right time. Artificial contraception involves the woman or man taking direct action to prevent the occurrence of pregnancy at which point sexual union  becomes a selfish act.
The Church also says that artificial contraception is morally wrong, because each and every sex act can occur only between husband and wife and must be directed toward two ends: love and life, that is, the intimate unity between the man and woman (love) and possibly procreating another human being (life). Conception and pregnancy don’t have to occur each time, but no man-made barriers should prevent what God may intend to happen.