Nestorianism is the belief that Jesus the God only possessed Jesus following his birth, that during the gestation period only Jesus the Human existed in Mary’s womb.
I don’t know too many people who even think enough about that today to have that type of belief, and if I found someone I think it would be easy enough to dissuade them of that viewpoint using biblical passages.
On the other hand, there are some who attribute extra-biblical attributes to Mary because of her honor of being the Mother of God. It is unfortunate, because the phrase “Mother of God” is useful in it’s own right, but some become scared of using it because of how others take it too literally (which was another heresy).
Nestorianism has nothing to do with the argument over whether Mary was sinless, whether she ever had sex, or whether she died.