[NSFW and content warning]

[I am so sorry if some of this comes across as insensitive.]

How often would priests be known to break the Church's rules on sexuality?

Looking at western Europe:

That part is pretty much "all the time," if you consider priests' and monks' (who were often ordained) vows of celibacy.

Throughout the later Middle Ages in particular, the Church leadership spends a good deal of time and energy yelling at priests in general to keep their celibacy vows. (Often while breaking them themselves, but you can't have everything.) The stereotypical image here is the village priests who lived with a "housekeeper," and possibly her children.

Everyone knew they were de facto married, but in most cases looked the other way. This was true to the extent that one of the Church's other rules meant to discourage priests from having sex was sometimes broken. Sons of priests were not supposed to be ordained. Yet there are plenty of cases where, yup, sons followed their fathers into the ecclesiastical profession just like so many other sons did for crafts or farming.

There was a legal system in place to regulate and punish this--ecclesiastical courts, though, not the civil/criminal ones. However, some really interesting studies for France have shown that priests actually charged with various violations, had generally aroused people's ire for some other reason. Having sex/having a "housekeeper" was in some ways a stand-in charge.

...But do not for one second believe the claim that priests sexually abuse(d) children because they were forbidden from marrying. Pedophilia apologists have NO place on AskHistorians.

[Were pedophile priests] at all a problem (or a well known one) in Medieval Europe?

The answer is almost certainly yes. But not in the same way it occurs today, and not with what is finally public documentation.

I want to be clear in advance that the first couple of issues I discuss are ones of medieval versus modern framing, not the abuse itself. I'm not trying to scorn any readers' pain.

The first problem is one of framing. Our age of childhood versus age of consent is (for current purposes) 18. In the Middle Ages, it was 12 for girls and 14 for boys. So it's kind of up to you to decide how you want to think about sexual abuse of medieval teenagers--how medieval society framed it, or how we understand more-or-less emotional maturity to fuzz in. (I realize that priests abusing teenagers is not our stereotype, but my understanding is that that account for a lot of the horror we know about.)

The second problem is labels. This relates to visibility in sources, NOT occurrence of crimes. In general--not just for the Church--sexual abuse of children was counted as a crime if it was intercourse, and then as "rape" with girls or "sodomy" with boys. If it was prosecuted at all.

Unfortunately, lack of outright visibility doesn't mean that sexual abuse of children never occurred. I don't believe I need to "prove" this, and you shouldn't either either. People in the Middle Ages didn't think it was okay, either. Studying child survivors of rape and sexual abuse in late medieval Iberia, Marie Kelleher emphasizes how much accused/suspected rapists of children were reviled.

But for more systematic historical analysis:

The worst way to excavate sexual abuse of children from sources, of the studies I've read, is to look at infanticide cases. An all too common reason that mothers give for their crime is that the baby was the result of her father or grandfather having sex with her. (And as Kathryn Gravdal points out--in these cases, it was the mother who suffered punishment for infanticide, not her rapist.)

So as awful as what I'm about to say sounds, talking about incestuous rape of daughters leads to a necessary point: opportunity.

This is also stressed by Marie Kelleher in her research on inquisition documents (the type of legal proceeding, not the Spanish Inquisition) from Iberia. She discusses accusations of rape of girls, and not priests. But their common denominator applies equally to the, uh, mix-and-match: the abuser drags their victim to someplace out of other people's sight. Kelleher's focus is children at work, either alone or in a group with other children--which is to say, away from other adult supervision.

That brings us to what I'm going to make my final point for today, because I can't handle anything more.

Kids aren't dumb. It's that pedophiles are manipulative. Holding a position of power over someone--especially a kid--is coercive through fear, through a kid trying desperately to please, through convincing the kid that abuse is normal and any discomfort is wrong. Today, as I understand it, that tends to mean teachers, coaches, similar mentor-type figures.

In a lot of ways, the medieval situation was different. A few more rural parish schools did exist in some places by the late Middle Ages. And city schools for middle/upper class boys and a few (often, not always informal) for girls were increasingly common. So you would have priests, or at least others in clerical orders (there were a bunch), as at least some of the teachers in boys' schools.

I'm guessing pedophile priests would have taken more advantage of one-on-one tutoring, going from Peter Abelard's account of his "instruction" of Heloise in the 12th century:

Thus, utterly aflame with my passion for this maiden, I sought to discover means whereby I might have daily and familiar speech with her, thereby the more easily to win her consent. For this purpose I persuaded the girl's uncle, with the aid of some of his friends, to take me into his household. [...]

When he had thus given her into my charge, not alone to be taught but even to be disciplined, what had he done save to give free scope to my desires, and to offer me every opportunity, even if I had not sought it, to bend her to my will with threats and blows if I failed to do so with caresses?

Even if Peter is altering some details of the overall story, his version of events shows a perfect understanding of how to craft a situation to sexually abuse someone.

Another major dangerous situation, one that doesn't even cross our minds today, was for child oblates in monasteries. If you want to talk about positions of power and coercion... There's a 12th century writer who makes a "joke" about this, even. (And no, I'm not going to repeat it here.)

Spiritual mentoring/counseling scenarios are also (I think?) a less likely scenario than today, simply by virtue of not existing very much. Confession was exclusively public. Visition records for dioceses (basically, outsiders checking to see if a church/pastor was doing its job) are...equivocal...on the religious knowledge of parishioners. Granted, the visitators often had their own agendas, but still.

Please don't think that list is exhaustive. Those are just some examples of situations where priests held definitive power over children and could manipulate them into situations without other adults present.

So there is no surviving shit ton of documentation of priests sexually abusing medieval children. We can't point to patterns of abuse, only speculate. We can't recover the experiences of survivors, or of kids who didn't survive. We can't talk about their long-term traumas.

But then, think how recently we learned about the extent of pedophile priests today. With the Internet. Versus a world where paper didn't even exist until the late 1300s, and we're lucky if we have baptism records for a single year in a given village church.

While I think u/sunagainstgold did a great job, I'd like to talk about this question in the context of one particular and very notable attempt at addressing this problem in the Middle Ages.

St. Peter Damian, dubbed the "Doctor of Reform" in the 19th century, was a prominent reformer in the 11th century who wrote many letters and treatises on the issues of sexual immorality in the Church, with a particular emphasis on sodomy and pederasty. Pedophilia in the Catholic Church was absolutely a problem in the Middle Ages, and I would argue that Damian made it more well known than the Church at the time would've probably preferred.

As the other commenter pointed out, the clergy were subject to a different set of laws than the laity. The clergy were generally exempt from civil law, and were instead subject to the Church's canon law. That said, the enforcement of that law was not always as strong as reformers like Damian would've preferred. Damian explicitly blames bishops and their lack of willingness to provide discipline, which he believes stems from a fundamental misunderstanding they had about their role in the Church. He believed bishops should act as teachers and spiritual guides, but many if not most bishops viewed themselves as princes or lords of the Church, treating Church property as if it were their own. This allowed them to rule as they saw fit, and the position was somewhat understandable as a vast majority of these bishops had bought their way into their position. Damian wrote letters to both Pope Nicholas II and Pope Leo IX to convince them to act on what he saw as a widespread problem, and neither Pope really did as he asked. Leo IX responded in a letter where he “appreciated and confirmed [Damian’s] findings of the continuous existence of deviant sexual behaviors and child sexual abuse within clerical ranks” (Rashid and Barron 2018) but he disagreed on how they should be handled. Damian believed offenders should be cast out of the Church, and Leo IX was hesitant to do so. He only deposed long-term repeat offenders.

Damian, understandably, grew very frustrated with the Church leaders and their refusal to address the problem himself. He shifted his tone around the mid-11th century, suggesting that if the Church could not reform itself, then the laity was responsible for reforming it. His treatises ended up being widely distributed and read among both the laity and the clergy, although we must remember that most of the population at the time was not particularly literate, so it's difficult to say how far his messages reached.

I would say it is difficult to estimate how large of a problem sexual abuse was in the Church since the stance of Pope Leo IX set a precedent for relatively lax punishments, and our only true records of these events transpiring come from others describing them or ecclesiastical court documents. Damian suspected, as do I, that the Church's primary motivation in not prosecuting these cases came from a desire to cover up the problem to keep the laity from judging the Church. Damian's adamant calls for reform and Pope Leo IX's response confirming that it was a widespread problem leads me to conclude that it was most likely worse and more widespread than officially recorded cases.

For Further Reading:

Anderson, C. (2004). When magisterium becomes imperium. The Journal of Theological Studies, 65(4), 741-766. doi:10.1177/004056390406500403

Rashid, F., & Barron, I. (2018). The Roman Catholic Church: A Centuries Old History of Awareness of Clerical Child Sexual Abuse. The Journal of Child Sexual Abuse, 27(7), 778-792. doi:10.1080/10538712.2018.1491916