Does this help. An extract from this link http://neoenglishsystem.blogspot.com/2010/09/women-in-prologue-to-canterbury-tales.html

In actual life, nuns were nearly always of gentle birth for a reason that was largely economic. Nearly every woman of the Middle Ages had her social "place", fixed from birth. If she belonged to the peasant or artisan class, she had little chance to remain single : her labour was her valuable dowry, and neither father nor guardian of any sort would be likely to withstand the pressure of a suitor. Indeed, a healthy girl of that social standing was a commodity, and although she might have some choice which man she wedded, the choice ended there, for her destiny was to be a married woman, not a nun. Upper-class women were in a different situation ; they could not 'labour', so their dowries could be only in money, or in the power of family connection. .If a girl were the daughter of a rich or influential house, she would be married out of hand at an early age, sometimes even by proxy in her cradle. Marriage was a business through which a man furthered his finances or his opportunities or advancement. If she came from an impoverished family...and many knights were from wealthy...the lady, if she wished to survive at all, became a nun. Today we are accustomed to think that the religious life is only for those who have a vocation, and we may wonder/about those medieval ladies who entered convents simply because there was literally nothing else for them to do in the hard, non-fairytale world of practical matters. Were they "good" nuns? Were they happy? For the overwhelming majority, the answer must be "yes" to both questions. As a young girl, the lady herself had probably been schooled by gentle nuns who had taught her all the polite accomplishments, as well as the practical arts belonging to her station. In fact, the life of the school girl in the convent was often more exciting and could be far more opulent than in her own home. Further, the nuns being medieval women, regard in the tradition of the medieval Church, would inculcate the Church's tenet that the virginal life was the ''best," the one most surely to be rewarded everlastingly in the life to come. Existence, then, within the convent's walls was busy, and it was peaceful, pleasant and dignified. One did not starve there or lack shelter, one was surrounded by one's social peers, and spiritually one was upheld by the supreme knowledge that one was a bride of Christ.