Jesus gave the Apostles the authority of binding and loosing (Matthew 16:19, Matthew 18:18).
This was Jewish terminology for establishing laws and norms for the people of God: forbidding (binding) and allowing (loosing) certain actions. The bishops (including the pope) are the successors of the Apostles. Therefore they inherited this power of binding and loosing. Thus, when the Church solemnly proclaims that we must not eat meat on Friday, the sin is not in eating meat, but in disobeying the legitimate authority of the Church.
Jesus told the Apostles,
|16 He who hears you hears me, and he who rejects you rejects me, and he who rejects me rejects him who sent me. (Luke 10:16)
The same applies to the bishops. Also, Scripture says,
17 Obey your leaders and submit to them; for they are keeping watch over your souls, as men who will have to give account.
Consequently, when our sacred pastors establish a norm for our spiritual good, especially when they consider it a grave issue, it is a sin to contravene that norm.
An argument can be made that Humanae Vitae is infallible. A good explanation is:
Zen is not compatible with Christianity. Zen is a form of Buddhism, which is a pagan religion incompatible with Christianity. Since Jesuit priests are Catholics and indeed Catholic clerics, for one to practice Buddhism is a scandal and an illicit form of syncretism and possibly idolatry (and/or) apostasy.