The reasoning is established by the scriptural quotes and by logic. Particularly, these days the habit of the sangha eating meat is unsightly to most people. It disgraces the teachings and is a cause of the arising of wrong views toward the Dharma.
The Buddha said, “If one eats meat it is a serious negative act. One should not take lives. Meditation on loving kindness and compassion is the heart essence of all the genuine teachings.” But some people do not understand this, and based on the behavior of some individuals wearing Buddhist robes they think, “The compassion of the Buddhists is merely lip service. They are meat-eating, people-tricking barbarians.”
Further some call Tibetans “cow-eating fools” or say “they speak about loving kindness and compassion, but in practice they eat meat and take lives.” For similar reasons others think that Buddhist monks are inauspicious and when they see them coming they quickly close their businesses.
In brief, by proclaiming such unpleasant things, people develop wrong views towards the refuge protectors of all beings, the only source of all happiness and benefit, namely the three jewels, and accumulate measureless negative acts. The genuine lamas of the past have considered this and spoken of it.
For example, regarding the sangha’s eating of meat, the divine lama Yeshe O said,
Less compassionate than flesh-eating demons
They like meat more than hawks and wolves.
If one becomes buddha by this behavior
Then hunters, fishermen, butchers and prostitutes
Without a single exception will certainly attain enlightenment.
From Je Changkya Rinpoche,
In the middle of a pile of flesh, blood and bones of animals
Killed for the sake of an offering to one,
Some hoist knives and act wild with desire
Like a host of guardians ready to subdue enemies.
Please look on these apparent sangha with compassion.
From Dza Paltrul Rinpoche,
These days flesh-eating demons [sangha who eat meat] have become the objects of offerings.
Mendong Tsampa Rinpoche said,
One orders the killing. One does the killing. One eats the meat. It is taught that these negative acts are all similar. One has whatever food one desires, but likes only the flesh and blood of their slaughtered domestic animals that have been equally kind as one’s parents. Some not only swallow, but ravenously eat it as if it were more delicious than other foods.
There are limitless such sayings.
Regarding Buddhists in general and monks and nuns in particular, by eating meat they not only accumulate negative deeds for self and other, but they disgrace the Buddha’s teachings as well. Abandoning eating meat yourself, with folded hands I request you to encourage others to do so as well by explaining the faults of eating meat, the benefits of abandoning it, and the reasons why it is not appropriate.