There were a number of reasons for women becoming nuns, either as young women or later in life (or, as in the case you mentioned of Marie Countess of Boulogne, both).

Later life Some women felt a genuine calling. Maud de Ufford, the widow of first William de Burgh earl of Ulster, then of Ralph de Ufford, justiciar of Ireland, entered religious life after the her second husband’s death. She claimed to have had a calling for some time, explaining in a petition to the pope that she had in childhood intended to become a nun. The implication is that her family’s plans for her marriage got in the way of this calling. In Maud’s case, the fact that her marriages had been fraught with drama – the death of each of her husbands saw her fleeing Ireland for her family in England, both times with a young child in tow – could well have been incentive to enter the relative stability of religious life.

This brings us to the major reasons that women entered religious communities later in life, particularly after a husband’s death. First, the medieval church really liked the idea of the ‘chaste widow’. Women’s sexuality was seen as a problem in general in the Middle Ages, but widows were particularly problematic, because they’d already experienced the pleasures of the flesh but were expected to remain chaste after a husband’s death – how could they possibly be trusted to do this? Joining a religious community was a fantastic way to keep tabs on a widow’s sexuality (though there were other ways, including remarriage or vows of chastity). There was even opportunity for extra heavenly reward: the Myrour to Lewde Men taught that a woman who kept a chaste widowhood would have her heavenly reward sixty-fold.

At the same time, widowhood made women vulnerable. Widows were (often for the first time in their lives) able to control their own financial and legal destinies, and many aristocratic widows were quite wealthy. This also made women vulnerable (as in the case of Maud de Ufford, above) and many women chose the cloister in order to achieve a degree of safety. As before, however, this wasn’t the only – or even the best – way of achieving this. Many widows chose remarriage or simply chose to remain single in the face of whatever threats they experienced.

And then, some women were pushed to enter religious life, though they didn’t always take religious vows. There is debate over whether Elizabeth Woodville, then the dowager queen of England, retired to Bermondsey Abbey after the accession of Henry VII out of her own volition or because the king pushed her to.

As girls It was more common for women to become nuns from a young age. The fact that girls often officially entered religious life (even if they didn’t take religious vows for several years) at the age of 12 – the canonical age at which a girl could marry – indicates that marriage vs. religious life was very much an 'either-or' proposition, at least at this age. Again, there were a couple reasons behind this. First of all, as before, we shouldn’t discount the idea that some families had a genuine desire to give a child to the church. That daughter could serve as a symbol of her family’s piety and pray for their spiritual well-being.

Women from the top of the social ladder could become abbesses at powerful convents and exercise a good deal of power themselves. For example, the list of abbesses at Barking Abbey, one of the most prestigious convents in medieval England, boasts women from the most influential families in the country. To have one’s daughter or sister rise to a position of influence in a powerful religious house was no bad thing for the medieval noble in secular society.

I think that the most important factor, though, was money. Marriages were expensive for the families of both parties and it could be cheaper to donate a daughter to a nunnery (or, indeed, a son to a monastery) than to arrange a marriage for her. A similar thing happened with boys. It was very common for younger sons to either join the church or simply not marry, which was cheaper for a number of reasons. The result was a number of girls being sent to a nunnery. If you look at large families (in England, the Nevilles, Percys, or Beauchamps are an excellent place to start) in the Middle Ages, it’s really common to see at least one younger child sent off to join the church.

As you say, there were surely men of slightly lower class whom these women could have been married, rather than being packed off to a convent. Kimberly Schutt has looked at this in the early modern to modern periods, concluding that women who married ‘below’ themselves experienced some very nasty social consequences that men who married ‘down’ did not. This would naturally have varied according to circumstance and personal inclination, but it could well have been seen as better that a woman enter religious life than marry too far below her station.

I hope this helps to answer your question, though I didn't reference any of the specific examples you asked about. Please do follow up!

FOOTNOTES Rowena E. Archer, 'Rich Old Ladies: The Problem of Late Medieval Dowagers', in Property and Politics: Essays in Late Medieval English History, ed. by Tony Pollard

Cordelia Beattie, Medieval Single Women: The Politics of Social Classification in Late Medieval England

Calendar of Papal Petitions

Sandra Cavallo and Lyndan Warner, eds., Widowhood in Medieval and Early Modern Europe

Robert R. Edwards and Vickie Ziegler, eds., Matrons and Marginal Women in Medieval Society

Jack Goody, The Development of the Family and Marriage in Europe

Barbara Hanawalt, 'Remarriage as an Option for Urban and Rural Widows in Late Medieval England', in Wife and Widow in Medieval England

S. H. Rigby, English Society in the Late Middle Ages: Class, Status and Gender

Kimberly Schutt, Women, Rank and Marriage in the British Aristocracy: 1485-2000: An Open Elite?

Fiona Harris Stoertz, ‘Young Women in France and England, 1050-1300,’ Journal of Women’s History