The story of Peter Abélard and Héloïse was considered one of the great love stories by medieval writers, which is funny considering it involves castration, theology, and the monastic life (although I guess you could argue that castration's the best guarantee of unrequitable love).
Peter Abélard was born in 1079 in France, and by the age of 22 he had set up his own very successful school of philosophy. In 1115 he began teaching at Notre Dame, where he met a young woman called Héloïse, the niece of one of the canons at the cathedral. She was about 17, and was unusually well-educated for a woman, and the two immediately hit it off. Abélard fancied her so much that he managed to persuade her uncle to let him move in and take on the role of Héloïse's tutor (on the pretext that he was struggling for money and would get a rent discount in return for his tutoring).
It wasn't long until Abélard's crush had turned into a full-blown affair, which he describes thusly: "Our speech was more of love than of the book which lay open before us; our kisses far outnumbered our reasoned words. Our hands sought less the book than each other's bosoms; love drew our eyes together far more than the lesson drew them to the pages of our text. In order that there might be no suspicion, there were, indeed, sometimes blows, but love gave them, not anger; they were the marks, not of wrath, but of a tenderness surpassing the most fragrant balm in sweetness. What followed? No degree in love's progress was left untried by our passion, and if love itself could imagine any wonder as yet unknown, we discovered it. And our inexperience of such delights made us all the more ardent in our pursuit of them, so that our thirst for one another was still unquenched."
Hot stuff. Perhaps unsurprisingly, when he eventually discovered what was going on, Héloïse's uncle was pretty miffed. Not long after, Héloïse discovered she was pregnant, and gave birth to a son called Astrolabe. To pacify her uncle, Abélard agreed to marry Héloïse, but secretly because being married would have damaged his career. Héloïse was reluctant, being unconvinced that a secret marriage would satisfy her uncle and also having a rather low view of marriage: she says in a later later to Abélard that "though I knew that the name of wife was honourable in the world and holy in religion; yet the name of your mistress had greater charms because it was more free. The bonds of matrimony, however honourable, still bear with them a necessary engagement, and I was very unwilling to be necessitated to love always a man who would perhaps not always love me." She felt, amongst other things, that a philosopher shouldn't be bothered by the petty distractions of domesticity.
When her uncle made their marriage public, Abélard encouraged her to go to a convent for a while (though he was unable to stay away from her, and describes their later suffering as just punishment for their not-very-nunlike activities in a quiet corner of the convent). Taking this as a sign that Abélard was trying to get rid of his niece, Héloïse's uncle took the obvious next step to protect her, and persuaded some relatives to sneak into Abélard's house at night and, er, steal his family jewels.
Apparently eunuchs found it difficult to make a successful career out of academia at the time, so Abélard was forced into a monastery, where he had a rough time of it - quite apart from the doctrinal disagreements which led to his being accused of heresy, the monks he ended up in charge of tried to murder him by poisoning his drinks. Héloïse was consequently forced to become a nun. She was not happy, and wrote some fairly explicit letters to Abélard, detailing her sexual frustration and her dissatisfaction at being forced into a life for which she had no sense of calling. Abélard wrote back, and gradually persuaded her that they should write to each other about matters theological, and their passionate (at least on her side) correspondence eventually subsided into a more sober discussion of the monastic life, and how best to run a convent.
Their letters, and Abélard's biographical History of my Misfortunes form the basis of subsequent retellings of their 'romantic' tale, although Abélard also wrote several more substantial theological tomes, including the controversial Sic et Non ('Yes and No') which was a compilation of contradictory quotations from early Church Fathers, attempting to disprove the popular assumption that 'the Fathers' spoke with one voice and agreed on everything.