"Sins of the flesh are the lightest sins, because the flesh is weak."
Source: Pope Francis's Candid Views on Sexual Morality - Crisis
"What! Do you say that [impurity] is a light sin? But it is a mortal sin: and if it is a mortal sin, one act of it, though it be only the consent to a wicked thought, is sufficient to send you to hell. Is it a light sin? Even the pagans held impurity to be the worst of vices on account of the miserable effects that it produces.
Seneca says: 'Immodesty is the greatest evil of the world;' and Cicero writes:
'There is no plague so fatal as bodily pleasure;' and (to come to the saints) St.
Isidore says: 'Run through all sins, you will find none equal to this crime.'"
-St. Alphonsus de Liguori
Source: The Complete Ascetical Works of St. Alphonsus de Liguori, Vol. XV,