Was it just a thing assumed in theological writing by monks who really probably had little experience with women and assumed it was so because women were as Eve therefore temptresses, and man was considered akin to reason as women were akin to emotions and animality, and because of similar opinions rooted in ancient Greek, Roman and patristic authority? (But still, how did those opinion come to be?) Did common people in contact with women really think this was the case? Did women? If yes, how?? I mean, is the current popular opinion that men are the ones who care and think about sex more than women just as misguided? I just don't understand; it seems such an obvious thing to me - obviously men are, generally, the hornier ones, no? How could something so obvious to me, so obviously stemming from unchanging nature and therefore arguably always being-so be perceived as something to be exactly the opposite case by medieval people? Thanks for any clearuppance :)
I can't speak to common people as a whole. However, religious people (both men and women) recognized the idea that women's sexual instincts cannot be controlled.
Thomas Aquinas, a prominent church scholar, addressed these ideas in his Summa Theologiae. Within this document Aquinas tried to "reconcile the teachings of Augustine and other Fathers with Aristotelian philosophy" (Blamires 91). Specifically, he struggled between the Aristotelian idea of women as a flawed prototype for the man and Augustine's assertion that all beings made by god are perfect. Ultimately he argues that Eve bears a greater responsibility for the Fall and thus women are more sinful. He also argues that women are designed with the purpose of procreation. In my personal opinion, the writings of Aquinas and others suggest that it was the culpability of Eve that resulted in the idea of women as more liable to fall into sin but an argument could be made that Greek and Roman ideals influenced the thought on women.
As a whole much of the Church leadership believed that women were more susceptible to sin, including sexual sin. In writing a paper on the beguine women, I found that many church leaders were incredibly stressed by uncloistered women, who they believed would fall into sin if not watched over. Prior to the beguines women had been part of the Cistercian and Premonstratensian orders. However, in both these orders women were slowly boxed out. In the Premonstratensian Order women were originally accepted by the orders founder, Norbert but later condemned. Beguine women experienced similar pressures, leading to the formation of beguinages which were essentially walled communities within cities for beguine women. This is in direct contrast to the earliest communities in which women frequently traveled. It seems this gradual removal from society was in part a function of anxieties that women could not be trusted to be cloistered.
Even among the religious men that supported the beguine community, the idea of the female as inherently sinful prevailed. Jaques of Vitry, a traveling preacher associated with the beguine Mary of Oignies, praised beguine women for overcoming their sinful nature. Jacques ideas on women as a whole generally echo Aquinas's and other Church fathers. He notes in one of his sermons that women are "roving and lecherous once she has been stirred by the devils hoe". His appreciation of Mary came as a result of his idea that she was strong enough to fight and control her inherently lustful nature.
Many beguine women had a similar perception of themselves. Beguine mystics such as Hadewijch felt that they were chosen by God to be a conduit of his word precisely because they had managed to dominate the naturally sinful nature of women.
All of this adds up to the general perception that women specifically needed to control their sexual urges. However I think it would be remiss to not mention that religious men were also seen as susceptible to bodily temptation, if not to the same level as women were. Specifically an English document detailing the Rules for an Anchoress argues that women must remained cloistered in part so that men will not be drawn into temptation by them. Earlier monastic orders had also had focus on removing oneself from the temptation of women. However, these ideas were less common and I would argue that in general women were seen as much more sexual.
I would suspect that our modern day perception of men as the horny ones comes from a later shift in culture, rather than any innate biological trait. However, I don't know enough to say when this shift occurred.
Apologies for not having more quotes or direct information. My library card seems to have run out so I can't access JSTOR right now.
Sources
Analysis of Thomas Aquinas's Summa Theologiae:
Blamires, Alcuin, Karen Pratt, and C.W. Marx, eds. Woman Defamed and Woman Defended: An Anthology of Medieval Texts. Oxford: Oxford UP, 2002. Print.
Creation of Beguinages and the Cistercian and Premonstratensian Orders
Neel, Carol. "The Origins of the Beguines." Signs 14 (1989): 321-341. Print.
Views of Beguines and Their Supporters on Women
Stoner, Abby. "Sisters Between: Gender and the Medieval Beguines." Ex Post Facto IV (1995): 26-51. San Francisco State University Journal Archive. Web. 26 Feb. 2015. http://userwww.sfsu.edu/epf/journal_archive/volume_IV,_no._2_-_sp._1995/stoner_a.pdf.
Rules for An Anchoress
Blamires, Alcuin, Karen Pratt, and C.W. Marx, eds. Woman Defamed and Woman Defended: An Anthology of Medieval Texts. Oxford: Oxford UP, 2002. Print.
I mean, is the current popular opinion that men are the ones who care and think about sex more than women just as misguided?
This is a "nature versus nurture" question -- how much of our gendered behavior is innate, and how much is learned or absorbed from our culture? AskHistorians doesn't generally allow too much discussion of stuff other than history, but if you start a thread about this at /r/AskSocialScience and/or /r/AskAnthropology you may get interesting perspectives from psychologists, sociologists, and anthropologists.
The consensus, as I understand it, is that our gendered behaviors are learned, rather than inborn! It's not my field and so I'm not really competent to give you a thorough overview, but my understanding is that because there is so much diversity in gendered behavior between different cultures and across different places and times, we have come to see these behaviors as culturally determined, rather than biologically determined.
There are dissenters, like the neurobiologist Louann Brizendine, who has tried to make a case that women's brains are structurally different from men's brains. It doesn't seem like she's taken very seriously, even within her own field. In fact, to me she seems like a crank -- but I'm not really qualified to judge. /r/AskScience would know more.
(edit for punctuation)