This is a very interesting question to consider, as it is possible to view the works of Marie de France as being a lot more radical than readers might first assume. There is a marked emphasis on love that is expressed in non-conventional ways, for example, and a high number of the narratives that Marie de France writes feature adulterous love, or love out of marriage, or even polygamy. However, what feminists would object to in her works is the presentation of courtly love as depicting women as objects who need to be "won" by men. A classic example of this would be in the tale "The Two Lovers," where a widowed king devises an impossible test to ensure that no prince can marry his beloved daughter. Feminists would no doubt baulk at the presentation of a woman as a "prize" to be "won" by completing a daunting and challenging physical task. This clearly objectifies women, and would be a massive objection that feminists would have to the presentation of courtly love in Marie de France's works.
However, it is important to consider the ways in which Marie de France subverts notions of courtly love. In this tale for example, instead of the traditional happy ending that we would expect most fairy tales to end in, the prince dies when he completes his task, having refused to take a potion to give him strength. Note what the narrator says about his refusal:
I fear the potion will do him little good
For he had within him no moderation.
The death of the young handsome prince therefore could be seen as an indirect criticism of the passionate nature of courtly love, and the way that it robs its victims of all sense and reason. Other readings of such works are always possible, and it is important to be aware that, much as feminists might decry the presentation of women as prizes to be won, there are other, deeper and more subversive readings of these works.