Guibert of Nogent called him "the liquid sacrifice to Satan." Columbanus recommended a two-year fast for this act, and Jean Gerson recommended self-flagellation as a cure. What's the matter?
The turn of the 4th and 5th centuries AD marks not only the decline of antiquity and the beginning of the Middle Ages. It is also the beginning of the Christian interest in the problem of masturbation. In the first centuries after Christ, the Church was preoccupied with more important problems (survival, persecution, fighting lions, etc., etc.), but as her position stabilized, thinkers began to discuss a problem that plagues many to this day. 
Saint John Cassian (d. 435) claimed that erotic dreams, nocturnal reflections, and masturbation were signs of lust and could become an obstacle even for a very pious monk on the path to holiness. Saint Caesar of Arles (5th-6th centuries) had a similar opinion. Guibert of Nogent (11th-12th centuries) went to the extreme in his views.
Sacrifice to Satan
He wrote a moralizing story about a faithless monk who wanted to explore the secrets of black magic. For help he went to a familiar (and how) Jew, who in turn agreed to introduce the (un) holy man to the devil himself. Satan, in exchange for access to secret knowledge, demanded from the man a sacrifice made of ... his sperm. "You sprinkle it on me, and then you taste it, for that is what must be done by worshiping me" - was to say the lord of hell according to Guibert of Nogent. It was this peculiar story that was then repeated by the priests from the pulpits, warning the faithful of the true purpose of onanism (glorifying Satan!).
Nudity in the Middle Ages was not as much over censored as we think it is today.
St. Columbanus, who lived in the 6th century, had a similar view on masturbation, and in his penitentiary he recommended that sinners guilty of this act be punished with at least two years of strict fasting. Same as for sex with animals.
Cardinal Sin
Perhaps the writings of Guibert and Columbus were read by Archbishop Reims Guy of Roye, who three centuries later declared that masturbation was a sin so grave that only the bishop himself could forgive it, and by way of exception . All these opinions, however, were somewhat radical. Outside of monasteries and centers of religious life, the problem of masturbation was approached as if ... it wasn't a problem at all.
No one publicly stigmatized the masturbators, nor did secular authorities try to hold them accountable in any way. Even the various authors of penitentials (penitential books) viewed masturbation very tolerantly.
Paenitentiale Bigotianum from the 9th century, it recommended that masturbating priests be punished with only a three-week fast . Old English Penitentiary “Paving his body with his own hands” also took only a 20-day fast.
One might think that the Middle Ages were… truly obsessed with penises! Two medieval brooches in the photos. Yes, people would go out on the street with something like that! (Photo:Journal of Archeology in the Low Countries).
Penalties were generally similar for men and women, except where the use of items was involved. For female masturbation with the use of an artificial member, Burchard from Worms (10th-11th century) recommended a year (if the game was played alone) or even a three-year penance (when more people participated).
Other scholarly work has emphasized that masturbation is the least serious of all sexual sins. Even in the church, people wondered if such an insignificant act required confession and penance at all.
What if… everyone does?
The broad medieval approach to masturbation was summarized at the turn of the 14th and 15th centuries by Jean Gerson, who devoted an entire scientific treatise (sic!) To it. On the one hand, he considered masturbation to be an important problem, and on the other, as a phenomenon that affects everyone from an early age. As James A. Brundage explains in his book "Law, Sex and Christian Society ..." Gerson was of the opinion that " Boys often start to masturbate as early as the age of five or even three . By the time they grow up to appreciate the importance of their act, they are already so addicted to self-pleasure that they are still manipulating their genitals passively. "
The solution to the problem according to a medieval thinker? Good company, lots of prayer, frequent fasting and self-flagellation.
How many people did a picture like this inspire sinful deeds? "Decretum Gratiani" with commentary by Bartolomeo de Brescia, Italy, 1340-1345 (source:public domain).
And if all this did not help - you could always spit on the ground, according to Gerson, while uttering the formula to ward off the devil. Lust should let you forget about itself for a while.